h1b延期时候离境是允许的。但问题不简单

如果extension还在pending的话，你在旧的 H1b失效前回来，能入境，但拿到的是只有剩下几个月有效期的I-94卡。回来后extension批下来的94卡上面附的原先94号码已经作废（根据最后原则），你还得再出境去拿一个新的3年I-94有效期，凭1年内2次入境，在签证有效期内可以免签证。
最好的情况是晚出境，等extension批下来后再入境。这时候你可以让别人寄给你新的h1b，凭其签证，拿到3年有效的94号码。

摘录一段：

Traveling while an H-1B extension petition is pending does not abandon the extension petition. However, please alert the University International Center of any travel planned during this period. If the extension is approved while you are outside of the U.S., it will be necessary to send you the new approval notice overseas. The latest approval should always be used when entering the United States. Otherwise, the I-94 will be issued with the end date of the previous approval and it will be necessary to exit and reenter the U.S. to obtain an updated I-94. This is because USCIS considers the last I-94 issued to be the current, active one.

If your extension is pending, you still need a valid H-1B visa and a valid H-1B approval notice to reenter the U.S.