The invention relates to the technical field of auxiliary installation for radar velocity measurement, in particular to a radar velocity measurement device with a modular assembling mechanism. The radar velocity measurement device comprises a radar velocity measurement device body with a radar velocity measurement system arranged inside, a longitudinal support rod with the bottom fixedly installedon the ground, and a horizontal support rod fixedly installed at one side of the top side of the longitudinal support rod by a bolt. According to the radar velocity measurement device provided by theinvention, a top mounting slot with open front and rear sides is formed in the upper surface of the horizontal support rod and a mounting base having the top with an overturning support tube is inserted into the top mounting slot; and an elastic top ball is controlled by a lock bolt arranged at the overturning support tube, so that the mounting angle of a fixing frame at the top of the upper endof the mounting base is fixed. The whole mounting mechanism employs an insertion mode from top to bottom, fixed plates are arranged at the two sides of the mounting base and a positioning block is arranged at the bottom, so that the overall stability is enhanced and the durability is high. Besides, the horizontal support rod is slotted to realize internal wiring and wire arrangement conveniently,so that connection and use become convenient.