At present, the resistance box and the electronic load used in the on-site detection of the EV off-board charger are large in size and heavy in weight which results in the limited use in some detection occasions. If the power battery pack of the mobile detection vehicle is directly used as the charging load, the charging mode is single and the electromotive force cannot be flexibly set, which results in long charging time and low efficiency. Aiming at these problems, this paper proposes a topological scheme that the lightweight controllable charging load consists of a vehicle-mounted power battery pack with a cascaded DC/DC converter,researches the double closed loop control methods such as constant input voltage, constant input current, constant input power and constant resistance of load, then achieves the typical working modes including constant input voltage,constant input current, constant input power and constant resistance of load. The operating modes has the ability to continuously adjust in the rated range. In a word, the proposed scheme can achieve self-test of the off-board charger while reducing the weight of the charging load. Finally, the simulation model in Matlab/Simulink is established to verify the feasibility of the proposed topology and the effectiveness of the control methods.