The invention discloses a welded steel pipe welding groove flatly-cutting device capable of being stably fixed. The device comprises a base, a lower rack, an upper rack, a left clamping block, a rightclamping block, a movable sliding frame, a driving motor, a cam and a cutting motor. The other end of a first telescopic rod is in threaded connection with the left clamping block, one end of a second connection rod is in threaded connection with a second telescopic rod, the other end of the second telescopic rod is in threaded connection with the right clamping block, an output shaft of the driving motor is provided with the cam forwards through interference fit, the movable sliding fame is installed on a second supporting column in a sliding mode, a cutting motor is fixedly installed insidea fixing support, and an output shaft of the cutting motor is connected with a cutting wheel. The left clamping block and the right clamping block are arranged to achieve the effect of clamping a steel pipe, the steel pipe is prevented from being shifted in the cutting process, on one hand, cutting precision can be ensured, on the other hand, the steel pipe can be prevented from being popped outto injure people in the cutting process, and therefore operation safety is greatly improved.