The present invention relates to the monitoring and detecting field of the running state of a motor and particularly relates to a portable motor efficiency on-site detection device. The device comprises a signal acquisition module, a processor module and a display module. The input end of the signal acquisition module is electrically connected with output end of the motor, and is used for acquiring the operation data of the motor. The input end of the processor module is electrically connected with the output end of the signal acquisition module and is used for calculating the operation efficiency of the motor in real time. The input end and the display module is electrically connected with the output end of the processor module and is used for realizing the display of the motor efficiency and the display of the man-machine interaction function. The invention also relates to a portable motor efficiency on-site detection method. According to the technical scheme of the invention, through acquiring the voltage signals and the current signals of the motor, the motor efficiency can be detected in the online manner. The limitation in testing the motor efficiency by utilizing a torque sensor, a rotating speed sensor or other devices can be broken through. Meanwhile, the device is convenient in installation and simple in structure.