The invention discloses a heat recovery system and method based on a bypass air preheater. The bypass air preheater is arranged at the part of a flue air preheater at the tail part of a boiler and is connected with an existing air preheater in parallel; the quantity of flue gas is adjusted according to the change in the quantity of the flue gas and the temperature of the flue gas at an inlet of the air preheater; the heat of the flue gas in the boiler is fully recovered through the flue gas distribution ratio of the existing air preheater and the bypass air preheater, so that the heat loss is reduced, and the boiler efficiency is improved. The heat recovery system and method disclosed by the invention have the benefit that the running mode of the system can be automatically adjusted according to the change in running conditions, so that the waste heat from the flue gas is fully utilized, the heat loss of the flue gas is recued, and the economy of the boiler is improved.