A method for determining the positions of laser beam has been introduced, in which a photodiode array combined with a CMOS signal processing circuit chip is used as a detector to measure the beam position with a large measurement range. Moreover, an interpolation method is used to improve the resolution of the photodiode array. With successfully measuring the beam shifts for the two kinds of laser alignment system, it is confirmed that the photodiode array and the interpolation method are available in laser alignment for long distance.