For a distribution network， to provide guideline for the preventive control before occurrence of violation and restorative control after faults， it is necessary to calculate the load margin real-timely and assess the security level on-line under various operating constraints. In this paper， the mathematical model of the load capability online assessment of a distribution network is proposed. By the repetitive power flow analysis algorithm， the load capability of a distribution network is evaluated in three perspectives: the entire distribution network， a subarea of the network and a load node. The security level of the current operating mode and the load transfer capability after faults are obtained by the proposed algorithm. By this mean， a scientific decision basis for the preventive control and restorative control are provided respectively. Simulation result shows that the proposed method is simple， fast and can be applied to distribution networks belonged to any voltage level. It can deal with all of the operating constraints and has good applicability.