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Testing of parallel programs involves two parts-testing of controlflow within the processes and testing of timing-sequence.This paper focuses on the latter, particularly on the timing-sequence of message-passing paradigms.Firstly the coarse-grained SYN-sequence model is built up to describe the execution of distributed programs. All of the topics discussed in this paper are based on it. The most direct way to test a program is to run it. A fault-free parallel program should be of both correct computing results and proper SYN-sequence. In order to analyze the validity of observed SYN-sequence, this paper presents the formal specification (Backus Normal Form) of the valid SYN-sequence. Till now there is little work about the testing coverage for distributed programs. Calculating the number of the valid SYN-sequences is the key to coverage problem, while the number of the valid SYN-sequences is terribly large and it is very hard to obtain the combination law among SYN-events. In order to resolve this problem, this paper proposes an efficient testing strategy-atomic SYN-event testing, which is to linearize the SYN-sequence (making it only consist of serial atomic SYN-events) first and then test each atomic SYN-event independently. This paper particularly provides the calculating formula about the number of the valid SYN-sequences for tree-topology atomic SYN-event (broadcast and combine). Furthermore,the number of valid SYN-sequences also,to some degree, mirrors the testability of parallel programs. Taking tree-topology atomic SYN-event as an example, this paper demonstrates the testability and communication speed of the tree-topology atomic SYN-event under different numbers of branches in order to achieve a more satisfactory tradeoff between testability and communication efficiency.