The invention discloses an SF6 insulation direct-current casing pipe fault diagnosis system, which comprises a preventive testing instrument, an on-line monitoring device, a fault diagnosis backstage and a communication device. The invention also discloses a diagnosis method based on the system. The method comprises the following steps: (1) extracting characteristic quantity of fault diagnosis of an SF6 insulation direct-current casing pipe; (2) establishing a diagnosis vector of the SF6 insulation direct-current casing pipe fault diagnosis system; (3) establishing a fault type incidence matrix of the SF6 insulation direct-current casing pipe fault diagnosis system; and (4) carrying out the fault diagnosis with the SF6 insulation direct-current casing pipe fault diagnosis system. With the adoption of the SF6 insulation direct-current casing pipe fault diagnosis system and the SF6 insulation direct-current casing pipe fault method, comprehensive and systematic fault diagnosis of the SF6 insulation direct-current casing pipe is carried out based on preventive testing data and on-line monitoring device data; and meanwhile, the condition that the fault diagnosis is carried out only aiming at preventive testing or on-line monitoring data at present is broken through, so that the overhaul ability of a direct-current system is improved.