The invention relates to the technical field of heat injection steam and thickened oil thermal recovery, in particular to a heat injection steam dryness testing device. The device comprises a condenser, wherein a steam injection pipeline is connected to a steam inlet of the condenser, a steam outlet of the condenser is connected with a steam discharge pipeline, a cold water inlet of the condenser is connected with a cooling water inlet pipeline, a hot water outlet of the condenser is connected with a cooling water outlet pipeline, and a flow control valve, a flowmeter, a thermometer and a pressure meter are arranged on each pipeline. By testing the physical property parameters of the inlet pipeline and outlet pipeline of the condenser, the dryness value of inlet heat injection steam is obtained through analysis and calculation. According to the heat injection steam dryness testing device and a dryness value calculating method, the state of steam in the steam injection process can be dynamically monitored in real time, the testing device is simple and compact in structure, online continuous measurement of dryness can be achieved, reliability and precision are high, the oil recovery ratio can be increased effectively, and the boiler tube explosion rate can be reduced effectively.