B2K singer Raz B nearly died Sunday in China -- after accidentally falling through a glass door in his hotel room ... slicing his wrist wide open ... and losing a SCARY amount of blood.

Raz -- who had been performing in Shanghai before the accident -- tells TMZ, he tripped hard inside his room ... and when he tried to break his fall, he crashed through a sliding glass door.

Raz says the broken glass caught his wrist and ripped it to pieces, tearing through ligaments, nerves, tendons ... right down to the bone. (We've seen a graphic close-up of the injury, and decided to spare you the visuals. You're welcome.)

Raz says he immediately began gushing blood and quickly passed out. Someone else in the room then alerted the hotel, and an ambulance was called.

Raz was rushed to Huashan Hospital Fudan University, where doctors operated to repair his mutilated wrist -- a grueling procedure that took roughly 5 hours.

Raz was released from the hospital Monday. He has since canceled his remaining tour dates -- for now.