盤前盤後可以下單，不代表適合下單：SEC 官方列的八個風險

一、時差把台灣人推向盤後

美股正常盤對台灣人來說是半夜。於是「盤前盤後可以下單」變成很多平台的賣點，也讓不少人把它當成正常操作。

可以做，跟適合做，是兩件事。 美國證管會（SEC）有一份專門的投資人公告叫 After-Hours Trading: Understanding the Risks，把風險逐條列出來。以下照原文拆。

二、先把時段講清楚

SEC 公告開頭寫著：

The New York Stock Exchange and the Nasdaq Stock Market—the highest volume market centers in the U.S. today—have traditionally been open for business from 9:30 a.m. to 4:00 p.m. Eastern Time.

正常交易時段是美東時間上午 9:30 到下午 4:00。 在這之外的就是盤前、盤後（統稱 extended hours）。

SEC 官方投資人網站 Investor.gov 的名詞解釋補了一句很實際的：

The duration of after-hours trading sessions varies between markets and trading venues. Investors should contact their brokerage firms to determine if and when after-hours trading sessions are available.

盤後時段的長度各家不同，能不能用、幾點到幾點，要問你自己的券商。 網路上任何一個統一的時間表都不能替代這件事。

三、SEC 列的風險（原文＋白話）

1. 看不到完整報價

Some firms only allow investors to view quotes from the one trading system the firm uses for after-hours trading. Check with your broker to see whether your firm’s system will permit you to access other quotes on other ECNs.

有些券商只讓你看到它自己用的那一個交易系統的報價。你看到的「最佳價」可能不是市場上的最佳價。

2. 流動性不足（Lack of Liquidity）

Liquidity refers to your ability to convert stock into cash. That ability depends on the existence of buyers and sellers and how easy it is to complete a trade.

白話：買賣的人少，想成交沒那麼容易。

3. 買賣價差變大（Larger Quote Spreads）

Less trading activity could also mean wider spreads between the bid and ask prices. As a result, you may find it more difficult to get your order executed or to get as favorable a price as you could have during regular market hours.

白話：買價和賣價之間的差距拉開，同樣一張單，成交價比正常盤差。

4. 價格波動（Price Volatility）

For stocks with limited trading activity, you may find greater price fluctuations than you would have seen during regular trading hours. News stories announced after-hours may have greater impacts on stock prices.

白話：交易量小的股票，盤後跳得更兇；財報和新聞常在盤後發布，衝擊會被放大。

5. 價格不具代表性（Uncertain Prices）

The prices of some stocks traded during the after-hours session may not reflect the prices of those stocks during regular hours, either at the end of the regular trading session or upon the opening of regular trading the next business day.

白話：盤後的價格，不一定代表隔天開盤的價格。

6. 多數只收限價單

Many electronic trading systems currently accept only limit orders, where you must enter a price at which you would like your order executed. A limit order ensures you will not pay more than the price you entered or sell for less.

白話：你可能只能下限價單。好處是不會買貴賣賤，代價是價格跑掉就不會成交。

7. 對手是專業機構

Many of the after-hours traders are professionals with large institutions, such as mutual funds, who may have access to more information than individual investors.

白話：盤後場上很多是共同基金這類機構的專業交易者，資訊條件跟散戶不對等。

8. 委託處理可能延遲或失敗

As with online trading, you may encounter during after-hours delays or failures in getting your order executed, including orders to cancel or change your trades.

白話：連取消和修改委託都可能卡住。

四、誰該注意

• 只能在台灣白天操作、習慣掛盤前單的人：你正好落在流動性最差的時段。
• 想搶財報行情的人：SEC 明說盤後新聞對價格的衝擊更大，而價格未必延續到隔天開盤。
• 習慣下市價單的人：許多系統盤後只接受限價單，操作邏輯要換。

五、怎麼做

1. 先問券商：有沒有盤前盤後、時段幾點到幾點、報價來源涵蓋哪些場所。
2. 一律用限價單，並且自己設定可接受的價格。
3. 把價差算進成本：盤後的價差本身就是一筆隱形成本。
4. 不急就等正常盤。這是最簡單、也最有效的風險控制。

本文整理 SEC 與 Investor.gov 公開頁面內容，不構成投資建議，也不推薦任何交易時段或商品。

站內延伸：小額交易成本見小額買美股，手續費可能吃掉你 3%，各家條件比較見券商排行與專欄總覽。

六、一句話結論

盤前盤後不是「多送你的時段」——SEC 官方公告列出流動性不足、價差擴大、價格波動、報價看不全、對手是專業機構等風險，而且許多系統只收限價單；能下單不等於划算。

實查：2026-09-07

資料來源（本日 VPS 直讀原始頁面）：

• 美國證券交易委員會（SEC）投資人公告 After-Hours Trading: Understanding the Risks：https://www.sec.gov/investor/pubs/afterhours.htm （HTTP 200）
• SEC Investor.gov 名詞解釋 After-hours Trading（正常時段 9:30–16:00 ET、時段長度依市場而異、應洽券商確認）：https://www.investor.gov/introduction-investing/investing-basics/glossary/after-hours-trading （HTTP 200）