The invention relates to an inner threaded part and an inner thread forming tool, and aims at solving the problems such as that an existing inner thread part and an outer thread part easily vibrate and loosen or lead to liquid leakage after being locked. The inner threaded part is characterized in that the pitch diameter of at least one inner thread is different from that of other inner threads, so that the inner threaded part can be clamped to the outer threaded part by screwing the at least one inner thread and the outer thread, and the inner threaded part and the outer threaded part are hard to loosen after being screwed, and as a result, the leakage preventing effect is achieved.