The invention discloses an inner tooth pre-positioning device and an inner tooth grinding method. The inner tooth pre-positioning device comprises a positioning support, a vertical spring, a horizontal spring, steel balls, a guide sleeve, a positioning disc and rolling bars. A gear ring with teeth to be ground is arranged on the positioning disc in a sleeving mode and makes contact with the upperend face of a tooth grinding support. The three rolling bars are arranged in an inner tooth groove of the gear ring along three arc grooves in the positioning disc. The gear ring is rotated, and thenthe rolling bars are pushed by an arc cam surface to achieve non-clearance positioning of the positioning disc and inner tooth pitch circles of the gear ring; and the positioning disc is pushed downwards to reach a tooth grinding position, and then tooth grinding can be conducted. According to the inner tooth pre-positioning device and the inner tooth grinding method, the procedure of tooth tip circle grinding does not need to be carried out, so that the production cost is reduced, and the production efficiency is improved; pre-positioning of the inner teeth is achieved in the inner tooth grinding procedure, and the link of manual alignment of tooth tip circles is omitted, so that the production efficiency in the inner tooth grinding process is improved; and meanwhile, the uncertainty of manual alignment is avoided, and the part machining quality is improved.