The invention relates to a water supplementing constant-pressure system of a pipe type heat exchanger and an abnormal operation control method. The water supplementing constant-pressure system comprises a nitrogen charging system which is used for maintaining the constant pressure of a whole pipe type heat exchanger system so as to avoid vaporization of heating medium water at the lowest pressure position and is started when the pressure in the pipe type heat exchanger system is too low, a water supplementing system used for supplementing the pipe type heat exchanger system for water in the water feed process and during water leakage of the pipe type heat exchanger system so that it can be ensured that a heating medium circulation system is filled with water, an exhaust system used for maintaining the proper running pressure of the pipe type heat exchanger system so as to ensure that the system does not exceed the pressure, and a draining system used for draining water when the liquid level of a heating medium water supplementing water tank is too high or during overhauling so that the influences on system functions due to the fact that water supplementing of the pipe type heat exchanger system is excessive and the water tank is excessively full are avoided. The water supplementing constant-pressure system of the pipe type heat exchanger has the beneficial effects that the closed type heating medium water supplementing tank is arranged, the whole pipe type heat exchanger is closed, scaling phenomenon on the heating medium water due to the fact that oxygen-containing gas enters the pipe type heat exchanger system can be effectively prevented. Due to the heating medium water supplementing constant-pressure system, it can be ensured that the whole heat exchanger system runs stably and safely for a long term.