The invention discloses a pressure test device for evaluating the road surface flatness and a pressure detecting method. The pressure test device comprises thin film pressure sensors, multi-channel signal collecting and modulating integrated machines and a computer / server; the thin film pressure sensors are connected with the multi-channel signal collecting and modulating integrated machines through anti-interference cables; when testing is conducted, firstly the pressure test device is placed in an engineering mechanical part, and double layers of rubber blankets are paved in a gap between the engineering mechanical part and a road surface; then round induction zone probes of the thin film pressure sensors are clamped between the double layers of rubber blankets, the channel signal collecting and modulating integrated machines record pressure data of test points where the sensors are located, and the computer / server processes time zone distribution features and frequency zone distribution features of pressure; and finally, a pressure root-mean-square value of each test point is extracted, a pressure value transverse / longitudinal distribution curve of each test point is drawn, and the road surface flatness is further evaluated. The pressure test device for evaluating the road surface flatness and the pressure detecting method are simple and convenient in operation, high in measurement accuracy, and wide in applicability, and have certain perspectiveness in monitoring of dynamic flatness of the road surface.