The invention relates to a telescopic cutting mechanism of a tunneling machine. The telescopic cutting mechanism comprises a cutting head, a working arm, a speed reducer, a cutting motor and a clasping device which are sequentially and coaxially connected from front to back. The speed reducer and the cutting motor are fixedly arranged in a telescopic arm, the telescopic arm is inserted into the front part of a fixed arm and is in front-rear sliding fit with the fixed arm, a propelling oil cylinder is installed inside the rear part of the fixed arm, a cylinder barrel and a cylinder rod of the propelling oil cylinder are connected with the fixed arm and the telescopic arm correspondingly, the telescopic direction of the propelling oil cylinder is in the front-rear direction, a guide flat keyis fixed at the rear end of the telescopic arm, a corresponding guide key slot capable of being in front-rear sliding fit with the guide flat key is arranged on the rear section hole wall of the front inner hole of the fixed arm, and the clasping device can be matched with the fixed arm in a conical surface inserting fit mode. The telescopic cutting mechanism is simple in structure, easy to machine and manufacture and high in reliability and maintainability.