To improve traffic efficiency, the intelligent transportation system (ITS) is widely used in urban roads. The sensing system is preferred to be deployed at roadside because the normal traffic would not be interfered. In the field of the magnetic vehicle detection, one of the most challenging problems is the disturbance from the other lane. Most existing magnetic detection systems need more than one magnetic sensor to solve the disturbance. This paper proposes a portable roadside vehicle detection system based on multi-sensing fusion. Cooperated with the magnetic sensor and the ultrasonic sensor, the disturbance from non-detected lane can be excluded and the traffic volume of the detected lane can be calculated accurately. Through on-road experiments, the vehicle detection accuracy can achieve 97.14% in a two-lane road. The experimental results demonstrate that the sensing system is able to be deployed at roadside in multi-lane roads to get traffic flow reliably.