A method for multiple sub-beams coherently emitting is put forward to improve the ability of laser to detect small targets in a long distance. The laser beam is split into many sub-beams with equal power parallelly emitted to the target in a certain arrangement. The sub-beams produce interference pattern on the target plane due to the fact that they are from the same laser source and have good coherence. Calculated results show that an interference peak is produced. Ideally, the maximum intensity would be N times as large as the intensity of the single beam emitted directly, where N is the number of the sub-beams. The detection is realized by using the interference peaks, and it will benefit especially to the detection of small targets. Beams divergence should be reduced to let a greater power to the target. In order to control the light intensity instability and the spot distortion, the jitters of the emitting mirrors must be controlled as well. Results show that a position accuracy of 0.1λ and an angle accuracy of 0.2θ are needed to have a stable and ideal interference peak, where λ is the wavelength and θ is the divergence angle of the beam.