The invention relates to an underwater metal fuel engine thrust testing device, and belongs to the underwater metal fuel engine technical field; two supports of the same structure are respectively arranged on two sides of a water tank; vertical support unit tops in middle of the two supports are respectively fixedly connected with two ends of a transverse support unit; the transverse support unit is horizontal; vertical support unit tops in rear portion of the two supports are respectively fixedly connected with a horizontal platform bottom; the platform is fixedly provided with a pressure sensor; the transverse support unit is in interference fit with a bearing seat through a bearing; one end of the bearing seat is fixedly connected with the top of a moment arm I; the other end of the bearing seat is fixedly connected with the top of a moment arm II; the moment arm I bottom is fixedly connected with the engine through a clamp; the engine is arranged in a water tank; the moment arm II bottom is fixedly connected with a cushion block; the cushion block tightly makes contacts on the pressure sensor; the moment arm II is horizontal. The underwater metal fuel engine thrust testing device can truly and reliably test the underwater metal fuel engine thrust force, thus solving the problems that an existing thrust test device cannot simulate a true environment and cannot accurately measure a minimal thrust force.