In order to realize THz gas spectroscopy system, researches upon off-chip measurement of 245 GHz subharmonic receiver chip are conducted. An off-chip measurement system for 245 GHz subharmonic receiver chip is established and an off-chip demonstrator measurement system for gas spectroscopy sensor based on 245 GHz receiver and transmitter chips is fabricated to measure the conversion gain and bandwidth of the 245 GHz subharmonic receiver chip. 15 dB conversion gain and 15 GHz bandwidth are obtained in off-chip measurement system, and 9 dB conversion gain and 16 GHz bandwidth are obtained in off-chip measurement demonstrator system. Experimental results indicate that the results of off-chip measurement system and off-chip measurement demonstrator system of 245 GHz subharmonic receiver chip are basically consistent. A THz gas spectroscopy sensor is constructed when a gas chamber filled with some specific kind of gas is added into the off-chip demonstrator measurement system. Compared with available gas spectroscopy of the same type, the subharmonic receiver chip has advantages of high gain, wide bandwidth, being integrated with local oscillation signals, and low power consumption, and is very suitable for applications of low volume intelligent gas spectroscopy in consumer electronics.