A pneumatic toy gun includes a pneumatic assembly and an air valve. The pneumatic assembly has an air intake duct to receive high pressure gas and includes a first duct, a rear push rod, a front push rod and a harness element. A first gas chamber is formed between the rear push rod and the first duct. A second gas chamber is formed between the front push rod and the first duct. The second gas chamber has a second thrust surface smaller than a first thrust surface of the first gas chamber. The front push rod has a recess. When the front push rod is moved and the recess is located beneath the first sealing element, the second gas chamber communicates with an air outlet through the recess. The pneumatic toy gun of the invention can provide a strong instantaneous blast force without increasing battery voltage.