The invention discloses a novel thread positioning device for a combustion chamber casing. A method comprises steps as follows: step 1, the thread positioning device is designed and manufactured; step 2, the thread positioning device is mounted on a to-be-machined workpiece; step 3, the positioning device is dismounted after a cutter is positioned; step 4, the thread size is machined and inspected. The fixed length of the device is integer multiples of the thread pitch of the workpiece, and the thread pitch and the thread shape are identical with those of the workpiece. The thread is machined by using the positioning device as the reference, a positioning line on the positioning device is aligned to a reference line on the workpiece during machining of the thread, the positioning device is mounted on the to-be-machined workpiece, a small carriage is moved, the cutter is attached to a thread groove in the positioning device, the position of the cutter is marked, the cutter tip is moved out of the position of the positioning device, and at the moment, the cutter is located on the workpiece and can accurately position the starting point of the thread. With the positioning method, problems that the thread of the engine combustion chamber casing is inaccurate to machine and position, the error is relatively large and uncontrollable, the replaceability of auxiliary products is poor and the production efficiency is low are solved.