Abstract: To ensure the reliability of laser firing, detection of all kinds of losses is necessary, such as the bend loss, dirty loss, nick loss, and ablation loss. A built-in-test (BIT) system with dual fiber optics and two lasers was founded by designing a dichroic coating and a dual-optical-fiber detector. The detected beam receiving rate of BIT system could reach above 6%. Further, the 808nm firing optical path loss could be deduced from the 650nm BIT path loss by transform of empirical formulae gotten by experimental study. Research results above could provide a built-in test method to laser ignition in engineering practice.