The invention discloses a boiler steam temperature adjusting system and a working method. The system comprises a superheater set and a reheater set; superheated steam of the superheater set and the reheater set and part of steam on a reheated steam outlet are connected to a low-temperature water heat exchanger which is connected to an economizer in parallel; when passing through the superheater set, the superheated steam passes through a superheat desuperheater, and part of the superheated steam is also connected into the low-temperature water heat exchanger; when passing through the reheater set, reheated steam passes through a reheating desuperheater, and part of the reheated steam is also connected into the low-temperature water heat exchanger; part of feed water passes through the economizer, and part of the feed water enters the low-temperature water heat exchanger; and in the low-temperature water heat exchanger, heat exchange is performed on the superheated steam and the reheated steam, and finally the superheated steam and the reheated steam are converged with an outlet of the economizer to flow to a next-stage heater. The boiler steam temperature adjusting system can balance temperature between different heating faces, the heat exchange amount, of radiation, convection and the like, between the different heating faces can be adjusted, adaptability of coal types and loads is improved, and energy losses in the temperature adjusting process are reduced.