Minimal receivable power of optical receiver is deduced by the means of detecting directly micro-pulsed laser signals, on the base of two statistic concepts: false alarm probabilities and detection probabilities of signal detection theory. The threshold selection has an influences decisively on the detection. Analysis on a practical system indicates that threshold correctly selected can minimize total misjudgment rate in the condition of meeting the system detection demands. At last, the minimal laser energies received with different laser pulse width are compared.