A white female Alabama cop has been placed on leave for laughing while using a stun gun on a crying handcuffed Black man ... with the distressing video widely circulating online.

In the clip, the unidentified cop from the Reform Police Department in Pickens County was filmed Saturday swearing and berating the man ... who District Attorney Andy Hamlin says was arrested on a trafficking fentanyl charge.

She's seen shouting at the cuffed man to lie on the front of the vehicle with his face down and ordering him to "stay still."

However, when the man responds, "I ain't doing s***, bro. I got a gun right there," she takes the gun out of his pocket and immediately deploys the stun gun into his back.

With the man clearly in pain and crying, she warns: "Do you want it again? Shut your b***h a** up!"

In response to the venom shown by the cop, reform police Chief Richard Black and Mayor Melody Davis released a joint statement ... reading: "The Reform Police Department is aware of a video circulating involving a citizen's arrest on December 2."

It was added: "The department is in the process of turning over all materials related to this arrest to the Alabama State Bureau of Investigation and has requested a thorough investigation into the circumstances surrounding the arrest. In accordance with City Policy, the officer involved has been placed on administrative leave while the investigation is conducted."

The clip has drawn widespread fury online ... with some labeling it "sickening" and an "abuse of power" and others calling for her arrest.

The identity of the man taken into custody on the drug-related charge has not been released.