The invention discloses a capturing method based on a signal light and a capturing system based on a signal light. A signal light capture technology is used, aiming is initialized according to the prediction position of the ephemeris, an optimal uncertain area is determined, a scanning path is set, each area in the uncertain area is scanned by using a fine tracking fast reflector, and image processing is performed by using a gray scale enhancement method, so that the signal-to-noise ratio is improved, the signal capture difficulty is reduced, and the capture time is shortened. According to theinvention, the communication beam, namely signal light, is used as an alignment beam; A beacon light transmitting system and a beacon light receiving system of an existing active communication terminal are omitted, a target or an indicator light of a reverse modulation terminal is omitted, the system structure is effectively simplified, the size, the weight and the power consumption of the systemare reduced, and the applicability of the free space laser communication system to a small platform and a movable platform is improved.