The invention provides a method for treating urban sewage with low carbon nitrogen ratio by using sludge liquor as a carbon source, which pertains to the technical field of water treatment in environmental engineering. The technical proposal of the invention is characterized in that higher-content organic matters in sludge liquor of excess sludge after thermal decomposition are used as an external carbon source; concentration of ammonian in the sludge liquor of excess sludge after thermal decomposition is lowered by an ammoniomagnesium phosphate settling method, thus raising the C/N ratio of the sludge liquor; and the sludge liquor is applied to sewage of lower total nitrogen deprivation rate due to shortage of carbon source by appropriate proportion, thus achieving the purpose of raising the total nitrogen deprivation rate. Compared with the prior art, the method has the advantages of saving cost on traditional carbon source, facilitating the operation, and having excellent environmental, economic and social benefits by a strategy of controlling waste by waste. Therefore, the method provides a final solution to thermal decomposition of the sludge liquor of excess sludge for sewage treatment plants.