Targeting at the ambient noise test of a large-scale fan, a sparse measurement method is proposed to deal with placing test-points. According to the acoustic and propagational characteristics of fan noise and the principle of sound pressure test, the sparsification of test-points is presented for optimizing the microphone locations. The outdoor ambient sound pressures of fan noise are tested at the sparse points and processed by time-frequency refinement analysis. As results of that, sparse measurement can obtain the messages of sound field effectively and thus get a comprehensive evaluation on the noise. This study extends the application of sound pressure test and provides a base for quantitative sparse measurement on ambient noises.