Turning to the new century，Chinese Social Security System has entered into a comprehensively developmental and creative stage with an aim to facilitate the integration of urban and rural development. This report summarizes the achievements of improving urban social security system，filling out the blank of social protection for urban and rural residents and resolving inherited problems. Then it points out that current system has realized full coverage for all people with increasing benefits and enhancing fund. Next，it analyzes the six main issues and four challenges facing current system. Last，it puts forward the suggestions to build a more fair and sustainable system.