In this paper, the problem about observation space of detector is discussed. A method of modeling the anti-models of ray-path from electric signal to parameter of ray-path using neural network is introduced. Making use of the special relations between the parameters of fitting lines at the crossover points of outputs of four-quadrant photodetecor and the three-dimensional free position, the distance between photosurface and focus of photodetector, the anti-model called model 1 of ray-path of four-quadrant photodetector is modeled. At the same time, a model called model 2 is modeled using the voltages of the output of photodetector at special time as observed quantities. Taking measuring for the distance between photosurface and focus of photodetector as an example, the measuring results are given, and the measuring results for repeatability of model 1 are also given. The measuring results for repeatability of model 1 show that the maximal measuring error is only 0.015 mm. And the measuring results show that the measuring accuracy of model 1 reaches micron level and that of model 2 reaches only millimeter level. Thus, that using the parameters of fitting lines at the crossover points of outputs of four-quadrant photodetecor as observation quantities is ahead of that using the voltages of outputs of photodetector at special time as observation quantities