The invention discloses a compact type constraint number gear shifting planetary gear train transmission, and belongs to the technical field of mechanical transmission and transmission design and manufacturing. The compact type constraint number gear shifting planetary gear train transmission comprises an input shaft, an output shaft and a plurality of planetary gear trains, and is characterized in that: a state that the degree of freedom is equal to or larger than 1 is constructed by changing the constraint number of the planetary gear trains, so that a fixed-transmission-ratio output gear or a neutral gear is obtained, when the degree of freedom of the planetary gear trains is equal to 1, a fixed speed ratio is output and corresponding gear output is formed, and when the degree of freedom is greater than 1, power cannot be output, thereby being equivalent to a neutral gear; and through parallel connection or series connection combination of the plurality of groups of planetary gear trains, gear output of different transmission ratios can be obtained. Gear shifting of the compact type constraint number gear shifting planetary gear train transmission is completed by controlling a brake, the clutch starting function is achieved, the types of parts are greatly simplified, and the gear train transmission has the advantages of being compact in structure, small in size, low in internal friction, light in weight, low in cost and the like.