The utility model relates to an on-line locating tube grade detection device for an electrified railway contact system detection vehicle; a plurality of electronic access detection devices are arranged in intervals on both sides of the centre point of the pantograph of the detection vehicle from the inside to the outside, and photoelectric distance measuring sensing devices with detection windows toward the locating tubes are respectively arranged at the points on both sides of the centre points of the pantograph, corresponding to the scope of the projection of the contact wire locating tube on the two sides. The distance measuring sensing device and the electronic access detection device are both connected to a data processing computer through a data processing unit and a communication unit. Based on the distance to the corresponding point of the locating tube, measured by the distance measuring sensing devices and the distance from the contact point of the pantograph and the contact wire at the locating point to the distance measuring sensing device, measured by the electronic access detection devices, the definite quality of the grade of the locating tube is obtained through the data processing computer. The device can detect the value of slope of the corresponding locating tubes passed by the pantograph in real time in the movement of the detection vehicle to provide references for the condition maintenance in power supply department.