Aiming at the test requierments of modulation transfer function for ultraviolet focal plane arrays, a system based on knife-edge scanning method is built to measure the modulation transfer function. Knife-edge is clearly imaged on the detector by a SW-Cassegrain reflective optical system and actual test shows that the optical system is near the diffraction-limitation in range of 0~100 lp/mm. An ultraviolet focal plane array device is tested by this system and the principle of maximum area under modulation transfer function curve is adopted to realize automatic focusing of knife edge image. The edge spread function curve is fitted by the model of three term Fermi function, which can effectively increase the fitting accuracy. Experimental results show that the measured modulation transfer function value of ultraviolet focal plane arrays is about 0.601, which is close to its theoretical limit. Meanwhile, the repeatability is good, and relative standard deviation of modulation transfer function whithin the Nyquist frequency range is about 1.8%. Uncertainty analysis indicate that maximum uncertainty of the equipment is about 6.2%, which can meet the needs of practical application.