The invention discloses a fault diagnosis method for a lower floating bearing of an axial flow propeller type hydraulic turbine generator unit. The fault diagnosis method comprises the following stepsof a, forming an observation port in a lower oil collecting basin during maintenance or shutdown of a unit; b, arranging an observation platform in a unit collector ring room; and c, observing oil stains on parts below the lower floating bearing through the observation port in the observation platform for estimating the oil leakage volume of the lower floating bearing. The method disclosed by theinvention is an observation method based on a fluid sealing technology, and additional installation of sensors and relevant software and hardware equipment thereof is not required; secondary damage to the unit is reduced, and the inspection and estimation are performed during shutdown of the unit, so that inquiry to staff by rotating components is avoided; and meanwhile the technical gap that thelower floating bearing in an oil supply head of the axial flow propeller type hydraulic turbine generator unit can not be monitored in the non-maintenance period is filed.