The invention discloses a device and a method for on-line detection of a tramcar wheel diameter with a sensor linearly and obliquely installed. The device comprises a trough rail, a processing center and a plurality of laser displacement sensors. The laser displacement sensors are connected with the processing center; the trough rail is a rail only keeping the trough part; the laser displacement sensors are arranged on the outer side of the trough rail one by one; sensing probes are evenly distributed in the same horizontal line under a wheel and measures the wheel upwards along different directions; detection light beams reach the wheel at the same time and are in the same plane with the circumference of the wheel diameter. The method comprises: the laser displacement sensors detect the wheel and obtain measurement points; coordinate transformation and data fusion are carried out; then the least squares fitting method is used to obtain the diameters and the centers of the initial fitting circles of a plurality of groups of wheels; then the genetic algorithm is used to calculate the diameter and the center of the optimal fitting circle; and finally the optimized wheel diameter of the tramcar is obtained. The device and the method have fast speed, high precision, large measuring diameter range and strong anti-interference ability.