A non-contact vehicle wheel alignment parameters measurement system based on line-structured laser sensors is described. The spatial position of vehicle wheel is determined by the tangent plane of the vehicle wheel tire. Three line-structured sensors conduct each wheel, and the tangent plane is calculated by 3D coordinate of each point on three characteristic curves of the tire. Consequently it is important to ascertain the tangent plane of the tire precisely for this system. This paper details the working principle of the system and coordinates unification technique of multiple line-structured laser sensors. Furthermore it presents a specific method for determining the tangent plane according to the characteristic of the laser image. It is composed of ellipse fitting in 2D space and searching algorithm in 3D space. Experimental results show that the algorithm is reasonably efficient, practical and reliable. And the deviations of alignment parameters are less than 2'.