美股实行的是T+3交割制度，即交易发生后第三个工作日才能完成清算交割。不过，美股市场允许投资者在满足一定条件下，在交割日到来前买卖同一支股票，从而实现T+0交易。

不过要做到美股T+0是需要满足一定条件的。这是因为频繁的交易可能会让没有经验的小额投资者很快损失掉资本，为了避免这种风险，美国证监会(SEC)制定了PDT (Pattern Day Trader)规则，对小额投资者的日内交易做出了限制。

美国证监会(SEC)官网上关于Pattern Day Trader的说明：

FINRA rules define a “pattern day trader” as any customer who executes four or more “day trades” within five business days, provided that the number of day trades represents more than six percent of the customer’s total trades in the margin account for that same five business day period. This rule represents a minimum requirement, and some broker-dealers use a slightly broader definition in determining whether a customer qualifies as a “pattern day trader.” Customers should contact their brokerage firms to determine whether their trading activities will cause them to be designated as pattern day traders.

A broker-dealer may also designate a customer as a “pattern day trader” if it “knows or has a reasonable basis to believe” that a customer will engage in pattern day trading. For example, if a customer’s broker-dealer provided day trading training to such customer before opening the account, the broker-dealer could designate that customer as a “pattern day trader.”

Under FINRA rules, customers who are deemed “pattern day traders” must have at least $25,000 in their accounts and can only trade in margin accounts. For more information on pattern day traders and related FINRA margin rules, please read the SEC staff’s investor bulletin “Margin Rules for Day Trading.”

美国证监会规定在五个交易日内如有四次或以上的日内交易(Day Trade)，且这些买卖占到同期总交易次数的6%或以上，即被认为是典型日内交易者(Pattern Day Trader)。

典型日内交易者的帐户资产(包括现金、股票或其它合规资产)必须达到$25,000。如果投资者交易行为满足了日内交易者的定义，而其帐户资产低于$25,000，则股票交易将被冻结90天，或直到投资者的帐户资产补足到$25,000。

相关信息 – 在中国国内如何买卖美股：

其实身处国内的投资者不出国也能在网上开户做空或买入美股、ETF（纽交所、纳斯达克或OTCBB上的股票），只要找能开国际账户的美国本土网上券商开立账户就可以（当然，这家网上券商必须是参加SIPC强制保险的，因为美国政府规定全部券商都必须是SIPC会员），具体开户步骤可参考这里：www.e-investingguide.com/openaccount

所谓日内交易(Day Trade)是指当日买和卖同一支股票，买进一支股票再在同一天之内卖出算一次日内交易。例如：当天分两批买入X股票，然后当天又一次卖出全部，计为2次当日交易。同时当天又买入并卖出Y股票，再计1次当日交易。

另外，美股帐户类型分现金帐户(Cash Account)和保证金帐户(Margin Account)。现金账户一般不允许成为典型日内交易者，要不受限制地开展日内交易需开设保证金账户。

综上，要在美股市场自由进行T＋0交易需开设保证金帐户，并且帐户资产要在$25,000以上(2000元以上，可开保证金账户，但T+0交易有金额限制）