The invention relates to an obliquely cutting device, in particular to a steel pipe obliquely cutting device. According to the technical problems to be solved, the steel pipe obliquely cutting device provided by the invention is simple in structure, convenient to operate and low in manufacturing cost. Through the steel pipe obliquely cutting device provided by the invention, the technical problems can be solved. The steel pipe obliquely cutting device provided by the invention comprises a bottom plate, a swinging seat, a motor I, a rotary disk, a concentric-square-shaped sliding rail, a sliding block I, a guide plate, a support rod I, support rods II, a support, an oblique sliding rail, a sliding block II, a connection rod, a guide sleeve, a motor II, a cutting piece, a clamping device, a fixed plate, a motor III, a support rod III and a collecting frame. The support, the swinging seat, the collecting frame and the support rod III are arranged at the top of the bottom plate sequentially from left to right. The guide plate is arranged on the lower portion of the right side of the support. The support rods II are arranged on the left side of the top of the guide plate symmetrically. The oblique sliding rail is arranged at the top ends of the support rods II and forms a specific angle. Through the steel pipe obliquely cutting device, the effects of a simple structure, convenient operation and low manufacturing cost are achieved.