A 400 kW radial-axial flux type experimental HTS synchronous motor is designed. There are twelve armature coils using HTS wires in the motor. They are accommodated in the cooling vessels made of FRP material, and twelve cooling vessels are enclosed in the vacuum vessel. In order to cool the HTS coils of the motor, a sub-cooled liquid nitrogen cryogenic system is presented. The operation temperature is below 70 K. This system consists of vacuum system, liquid nitrogen dewar, data acquisition system and a cold box with liquid nitrogen pump, G-M cryorefrigerator, cryogenic valve and a heater inside. The heater can present the heat of the HTS coils. In this paper, the design and the composition of this test cooling system are presented. When liquid nitrogen is injected into the vessel and the cryorefrigerator is operating, the cool down curve of the test system is obtained. After adding heat to the system, the capacity curve of the system is presented in this paper. The results proved that this sub-cooled liquid nitrogen system can be used for cooling the actual armature windings in the HTS motor.