The incident occurred at about 6pm at Kathu Shooting Range, located off Phra Barami Rd on the way to Patong.
Kathu Police were notified of the incident at 6:03pm and arrived to find that the injured woman had already been transported to Vachira Phuket Hospital by staff at the range.
The victim was later identified as Xiao Lili, 52, a Chinese national, who sustained a gunshot wound to her right cheek, with the bullet lodged inside. Her condition was reported as serious.
According to the initial investigation, Ms Xiao had arrived at the shooting range at about 5:20pm with a Chinese friend and used the facility under staff supervision.
After completing their shooting session, the pair were taking photographs in front of the shooting lanes when the incident occurred.
Police reported that a taxi driver waiting to collect the pair was alerted to the injury and, together with a range employee, rushed Ms Xiao to hospital.
Officers investigating the scene identified the suspected cause of the shooting as an accidental discharge of a firearm by a range employee.
The employee, named as Patiharn Manosong, 28, admitted to police that he had been standing in front of the staff room, about 20 metres from where the victim was shot, when he took out his personal firearm to inspect it.
He told officers he had forgotten that a bullet remained in the chamber, causing the weapon to discharge.
The bullet travelled through a wooden wall and a glass door before striking Ms Xiao in the face.
Kathu Police have taken Patiharn into custody and are proceeding with legal action. He faces charges of negligent conduct causing serious injury to another person and possession of a firearm and ammunition.
Police said further investigation is ongoing.