The increase of communication scale and non-deterministic communication make the communication test of Message-Passing Interface (MPI) parallel programs more difficult. In order to solve the problems, a new method of reducing communication coverage based on dominant relation was proposed. Firstly, based on the correspondence between communications and communication statements, the reduction problem of communications was converted into a reduction problem of communication statements. Then, the dominant relation of statements was used to solve the reduction set of communication statement set. Finally, the communications related to the reduction set were selected as the targets to be covered, so that the test data covering these targets covered all the communications. The proposed method was applied to 7 typical programs under test. Experimental results show that, compared with the test data generation method with all communication as coverage targets, the proposed method can reduce the generation time of test data by up to 95% without reducing the coverage rate of communications, indicating that the proposed method can improve the generation efficiency of communication coverage test data.