The invention relates to a variable-speed transmission mechanism, a variable-speed rotary cultivator and a speed change method, aims to solve the problems in the prior art that a high-and-low-speed rotary cultivator is inconvenient to operate, and provides a rotary cultivator capable of preventing spontaneous out-of-gear. The variable-speed rotary cultivator comprises a gear shifting mechanism which can freely adjust high and low speeds, the gear shifting mechanism is self-locked by virtue of a spherical surface, and after gear shifting, the gear is stable, and the phenomenon of spontaneous out-of-gear or machine stuck under an impact is prevented. The variable-speed transmission mechanism, the variable-speed rotary cultivator and the speed change method have the following beneficial effects: the structure is reasonable, the performance is stable, and the practicability is high; the gear shifting mechanism adopts volute splines for connection, so that gear combination is stable; and gear shifting can be directly realized without stops during operation of the machine, so that both time and labor are saved, the operation is safe and reliable, and the variable-speed rotary cultivator is trusted by farmers.