The invention discloses an automatic 
sand blasting device for the surface of a large flat mechanical part. The automatic 
sand blasting device comprises a main box, a 
sand blasting cavity is formed inthe main box, the left side of the sand blasting cavity is communicated with a left-axis sliding cavity, the left side of the left-axis sliding cavity is communicated with a transmission box sliding cavity, the right side of the sand blasting cavity is communicated with a right-axis sliding cavity, the right side of the right-axis sliding cavity is communicated with a stepping box sliding cavity,a transmission 
bevel gear box is in slidable and matched connection in the transmission box sliding cavity, and a transmission 
bevel gear cavity is arranged in the 
bevel gear box. According to the device, automatic sand blasting is conducted instead of manual sand blasting, the sand blasting efficiency is greatly improved, the harm, caused by the dust generated during sand blasting, to the 
human body is prevented, and the 
operation safety of operators is improved. Because sand blasting operation is carried out in the closed sand blasting cavity, the dust and 
waste gas generated during sand blasting cannot pollute the 
working environment of workers, and sand is more uniformly blasted to the surface of the part due to fixed-distance sand blasting, the problems of repeated blasting and blasting missing are solved, and the sand blasting effect is greatly improved.