The invention provides a pulse modulation signal phase noise measuring device. The pulse modulation signal phase noise measuring device comprises a down conversion and orthogonal phase discrimination arithmetic unit, wherein the down conversion and orthogonal phase discrimination arithmetic unit comprises a down conversion unit, an orthogonal phase discrimination unit and a sampling arithmetic unit, which are sequentially arranged from an input end to an output end, the down conversion unit comprises a local oscillator, a frequency mixer, a tunable band pass filter and a primary low noise amplification unit, the orthogonal phase discrimination unit comprises a phase discriminator, a low pass filter, a secondary low noise amplification unit, a loop filter and a reference source, the sampling arithmetic unit comprises an A/D (analog to digital) sampling unit and an analysis and arithmetic unit, and a phase noise measuring result is obtained after an output signal of the orthogonal phase discrimination unit passes through the A/D sampling unit and the analysis and arithmetic unit. The pulse modulation signal phase noise measuring device decreases numbers of needed test instruments and electric cables, simplifies measurement steps, and reduces test difficulty.