The invention discloses a water injection well ball throwing type downhole power generation device and method. The device comprises a main cavity, and a generator set; a water injection channel and aneccentric type electric generator cavity are arranged in the main cavity, the generator set is located in the eccentric type electric generator cavity, the upper end of the eccentric type electric generator cavity communicates with the position above an upper port of the water injection channel of the main cavity through an independent channel, the position, close to the bottom, of the eccentrictype electric generator cavity is provided with a radial water outlet hole communicating with the water injection channel, the upper end of the main cavity is connected with an upper joint, the upperjoint is provided with a main channel and a side flowing channel, the main channel communicates with the water injection channel in the main cavity, below the main channel, a ball base capable of bearing soluble balls is arranged in the main channel, and the independent channel in the upper end of the eccentric type electric generator cavity is the side flowing channel. Long-term stable power supplementing can be provided for the downhole intelligent water injection tool.