回答（仅供参考）：
It is not always mandatory that you should have SRS document to prepare a Test Plan.
This kind of Documents Hierarchy is maintained to maintain Organizational standards and also to have clear understanding of the things.
Yes you can Prepare a Test plan directly without SRS, When the Requirements are clear with your clients,and when your URD(User Requirement Document ) is supportive enough to clarify the issues.
Though we don't have SRS clients will be giving some information SRS only contains mainly Product information
But we will not know the Testing effort if we don't have SRS.
SRS contains How many cycles we are testing, and on the platforms we are testing , etc.
Actually there won't be any harm in doing so, becoz, ultimately you will send your Test plan document to your client and after getting approval from him only you start Testing.
(Note:-
SRS is the document which you get in the Analysis phase of your Software Development.
Test plan is the document , which contains the details of Product interms of , Tset strategy , Scope of testing, Types of tests to be conducted,Risk Managemnet , Mention of Automation Tool ,About Bug tracking Tool, etc..,)
我的思考：
我一直是比较赞同回答中的观点的。按照一般的思路，没有SRS文档怎么开始系统测试，怎么开始写系统测试计划，但我们不要忘了，SRS文档只不过是对需求分析的一个记录罢了，没有SRS文档我们仍然有其它的方式来弄清楚被测对象应该要做成什么样子，被测软件应该具有哪些特性。知道正确的、正常的流程是没错的，但我们不要把他看成死的了，要针对特定环境来进行变通，但目的只有一个那就是尽可能研发出好的软件来，而做测试就是尽可能在当前环境下把测试做好。
欢迎大家讨论！作者: 第十二颗地雷 时间: 2005-11-24 09:02
我觉得不是没有SRS的问题，而是测试计划的制定小组有没有相应经历的行业专家的问题！！
测试是一个综合性的工种，需要各方面的人才，才能够保证测试的完备性！！
不知楼主怎么看？作者: confusion 时间: 2005-11-25 13:16
“Yes you can Prepare a Test plan directly without SRS, When the Requirements are clear with your clients,and when your URD(User Requirement Document ) is supportive enough to clarify the issues.”作者: sunkitty 时间: 2007-4-22 19:47
这都是很灵活的！呵呵