The victim, believed to be Vietnamese, was injured at about 1430 this afternoon at the site in Waldram Park Road, Sydenham, in a “collapsed excavation”.
A Met Police spokesman said: “The man was pronounced dead at the scene. The Health and Safety Executive have been informed of the incident.”
Both an air ambulance and road ambulance attended but were unable to save the man.
A London Ambulance spokeswoman said: “Despite extensive efforts to resuscitate the patient, he was pronounced dead at the scene at 1510 BST.”
A Health and Safety Executive spokeswoman said a police inspector had told her the victim was of Vietnamese origin and had been killed by a collapsed excavation.
She added: “The police are making inquiries. It is too early to add anything more.”
The victim worked on the site but it is not yet clear who he was working for or who the owners of the site are.