In China, the generally small size and weak financing capacity have affected the growth of the securities companies. In recent years, many securities firms have adopted mergers and acquisitions (M&A) to expand the size of their businesses and it is necessary to study the impact of M&A on their long-term performance. This paper selects the acquisition of Guangzhou Securities by CITIC Securities as a case study to examine the impact of M&A on long-term performance. The paper first analyses the motivation for the M&A and describes the background of the two companies. Then it analyses the long-term performance of CITIC Securities before and after the acquisition. As for the financial indicators, the paper compares profitability, solvency, operating efficiency and growth capacity. In terms of non-financial indicators, it focuses on market share and number of business offices. Finally, it makes some suggestions for M&A in the Chinese securities industry. The paper finds that CITIC Securities' profitability, solvency and operating efficiency have all improved after the M&A, and the M&A has brought considerable gains. Its business offices in Guangdong operations have increased, and its market share has further expanded. In the future, CITIC Securities will need further follow-on integration to expand synergies and improve the company's performance.