The invention discloses a vehicle-mounted real-time road slope testing system and a testing method thereof. The testing system is characterized by consisting of GPS (Global Positioning System) testing equipment, a wheel speed sensor, an acceleration meter group and a data acquisition and processing plate card, wherein the acceleration meter group is used for testing the acceleration in different positions; the GPS testing equipment is used for testing kinematics parameters at a point A on a vehicle top plane; the wheel speed sensor is used for detecting the vehicle wheel angle speed in real time; the data acquisition and processing plate card is used for receiving the kinematics parameters at the point A, a square wave signal of the vehicle wheel speed transmitted from the wheel speed sensor and the acceleration measured by the acceleration meter group in real time; the road slope is obtained through calculation and is output through a CAN (Controller Area Network). The testing system and the testing method disclosed by the invention can avoid the limit as being integrated in vehicle-mounted testing equipment in artificial on-site operation, are high in inference resistance and are applicable to long-time slope data acquisition in a vehicle-mounted environment.