The invention relates to a method for preparing Portland cement, which uses a ball mill, which includes a bottom plate, a mounting plate, a grinding device, and a wall scraping device. The specific method flow for preparing Portland cement by using the ball mill is as follows: Clinker preparation, calcined clinker, clinker cooling, cement molding and inspection and packaging, grinding device includes drive motor, cylinder, inner lining plate, feeding pipe, material inlet, material outlet, material rack, compartment plate and steel The ball and the wall scraping device include a support rod, a lifting cylinder, a connecting rod, a scraping head, a fixed rod and an auxiliary mechanism. In the present invention, when the raw material is ground into powder through the grinding device, the powder adhered to the inner wall of the cylinder is scraped off in time through the wall scraping device to prevent waste and at the same time avoid the impact of the buffering effect on this part of the powder Reduce the grinding effect of steel balls. In addition, the wall scraping device can also regularly clean the scraping head to prevent too much powder adhering to the scraping head from affecting the scraping effect.