The invention discloses an online self-adaptive penetration detection device and method of laser welding. According to traditional detection, when welding parameters are changed, the position of a penetration characteristic area can be obviously deviated, a measurement position cannot be effectively corrected, and the test reliability is directly influenced. The detection device comprises a lasergun (5), the side surface of the laser gun is connected with the horizontal end of a right-angled adapter (12) through a bolt, the vertical end of the right-angled adapter is connected with a light filtering device through a bolt, an optical filter (11) is arranged in the light filtering device, the inclined end surface of the right-angled adapter is connected with an inclination angle adjusting flange seat (16) through a bolt, a spherical slide holder (17) is mounted at a seam allowance of the front end face of the inclination angle adjusting flange seat, a groove is formed in the middle of the spherical slide holder, a 45-degree reflector (13) is embedded in the groove, an adjusting screw (14) is connected with the inclination angle adjusting flange seat through threads, and a spring (15) is mounted in a hole of the inclination angle adjusting flange seat.