The invention provides a construction method for dismounting a shield tunneling machine in a tunnel. The construction method comprises the steps of hanging beam installation, conveying device preparing, feeding of the conveying device into a shield bored tunnel, cutterhead dismounting, lifting and conveying, tunnel upper half part dismounting, lifting and conveying, screw conveyor dismounting, lifting and conveying, main drive overturning support installation, cutterhead motor, speed reducer, propelling oil cylinder and personnel bin dismounting, lifting and conveying, shield tunneling machine main drive dismounting, lifting and conveying, splicing machine dismounting, well support and cantilever beam dismounting, lifting and conveying, dismounting, lifting and conveying of the lower half parts of the rest of the shield, splicing machine lifting and conveying, and trolley and back-up component separating and transferring. According to the construction method, a special hanging beam is adopted, a special conveying device is arranged to implement dismounting and transporting of the shield tunneling machine in the tunnel, the effect of machine dismounting in the tunnel achieves the effect of machine dismounting in a well opening, the completeness of the whole machine is good, the dismounted shield tunneling machine can be assembled again to be continuously used, and the problem that after an existing shield tunneling machine in the tunnel is dismounted, the completeness of the whole machine is poor is solved.