The invention relates to a testing system and method for the mechanical efficiency of a gearbox on a vehicle. Four wheels of a testing vehicle are each connected with a TPMS, and each TPMS is connected with a TPMS receiver; a first K-type thermocouple is connected with a hub data acquisition instrument; a second K-type thermocouple is connected with a transmission interface of the testing vehicle,and a signal is transmitted to the hub data acquisition instrument. A current sensor and a voltage sensor are connected with a power generator; the TPMS receivers, the hub data acquisition instrument, the current sensor and the voltage sensor are all in signal connection with a controller. The whole-vehicle gearbox mechanical-efficiency testing on a hub test bed has the advantages of convenient operation, controllable loading, reliable testing results and provision of the third party data collection. The mechanical efficiency study uses the whole gearbox as an object, the characterization accuracy is high, the test reliability is improved, and the system and method has important guiding significance for the development of gearboxes.