An adjustable sand blasting gun has a body, a sand inlet and an adjusting device. The body has a chamber, a blasting head, a mounting tube and a connecting pipe. The blasting head is attached to the body and communicates with the chamber. The mounting tube is formed on the body. The connecting pipe is formed on and laterally extends from the mounting tube. The sand inlet is attached to the mounting tube and has a passage and a through hole. The passage is formed through the sand inlet and communicates with the chamber. The through hole is formed through the sand inlet and communicates with the connecting pipe. The adjusting device is connected to the body and the sand inlet and has an adjusting post and a mounting pin. The adjusting post is connected to the connecting pipe and the sand inlet. The mounting pin is connected to the adjusting post.