The basic characteristics of incident spots and effects of external space environments on a four-quadrant detector were researched when it was used to detect the spot center. The formulae of the output and sensitivity of the detector were proposed when the incident spots showed an elliptical Gaussian-distribution. An experimental system was set up to test and verify the characteristics of the quadrant detector in an indoor test and a field test. Experimental results demonstrate that the detector has higher detection accuracy and subdivision in the indoor test, and can offer a 36-subdivision with digital filtering at a limited sensitivity. By the voltage characteristic curves, the background lights, spot sizes and the ratio of signal to noise were studied. The experiments in the field were also completed between the two spots of 830 m and 12.5 km. It is pointed out that the influence of irradiance scintillation on the detector is rigider than that of the indoor test. When the refractive index structure parameter is 10－16, the subdivision is only 7 in a distance of 830 m, furthermore, it can not detect spots in 12.5 km for the subdivision is invalid.