The invention relates to a system for preventing a sewer pipeline of a building from being blocked, used for solving the problem of blocking and backflowing of the sewer pipeline. The technical scheme thereof is as follows: the system comprises a main pipe of the sewer pipeline, consisting of a vertical pipe, a horizontal pipe and a turning elbow communicated with the vertical pipe and the horizontal pipe, wherein the horizontal pipe is led to a sewage well; after improvement, two ends of the turning elbow are connected with a distribution branch pipe in parallel; and an inlet end of the distribution branch pipe is positioned on the vertical pipe and an outlet end thereof is positioned on the horizontal pipe. In the system, an alarm device which can find the blocking of the pipeline in time is arranged on the sewer pipeline, and simultaneously the distribution branch pipe is arranged at the position of the main sewer pipeline so as to keep normal water flow state of the pipeline temporarily, thus effectively preventing the occurrence of the condition that the sewage returns upwards and overflows due to the blocking of the main pipe, avoiding adverse consequences and solving the worries in the future for residents at the downstairs of the building.