Abstract: The threshold algorithm of the target detection is often used to detect target echo signals.Its performance is based on signals-noise ratio.When signals-noise ratio is great than 6.7,the target can be detected.In order to increase the signals-noise ratio,matching filter is often used.If the system is narrow band system,the noise is color noise and matching filter cannot be used.Therefore,the signals-noise ratio cannot be increased.However,the geometrical characteristic of the laser echo signal is different with the noise.This paper advises the algorithm that detects the target by the geometrical characteristic.When signals-noise ratio is great than 2,this algorithm can detect target.This algorithm has been used in practice.