From the point of view of relativity principle and Doppler effect, the formula of changing value in receiver frequency for double antenna directional locating device is derived,. Under the circumstance of ignoring high order-small, it shows that Doppler frequency deviation is the sine function of angle between laser emitting direction and reference direction of receiver. At some special location, frequency deviation is zero. According to this location of "zero point", the danger signal direction of laser can be determined.