The invention discloses a device for changing automobile engine compression ratio, and aims to solve the problem of complex structure in the prior art. The device comprises a crankshaft, a first gear, four second gears, four third gears, a shaft, ten shaft seats, fourth to ninth gears, a sleeve and four rod journal sleeves, wherein the shaft 13 is fixedly to the lower part of the crankshaft through the ten shaft seats; the sleeve is sheathed on the right end of the shaft; the sleeve meshes with the sixth gear below the sleeve; the fourth to ninth gears are respectively sheathed on the shaft between every two adjacent shaft seats; the fourth gear and the shaft form a key connection; the other gears and the shaft form a rotating connection; the fourth gear, fifth gear, seventh gear, eighth gear and ninth gear sequentially mesh with the first gear and four third gears installed on the crankshaft; pinion gears of the four third gears sequentially mesh with the second gears which are installed on the outer end of each rod journal in the crankshaft; and the four rod journal sleeves are sheathed on the four rod journals in a key connection mode.