I was looking into baidu and saw some spoilers for this season:
"(Google translated): Luo Yingzi, Qiu Hua, and Xia Shu were swept out by the Liangcheng law firm. Luo Yingzi suspected that the incident was related to a old case she was checked by Mei Liang. In order to survive, the three started a law firm. In order to pursue Luo Yingzi's job change to the Liangcheng law firm, Chen Shuo fluttered. Luo Yingzi still did not accept Chen Shuo, but was fascinated by Xu Zhuo, a new partner. Chen Shuo secretly helped Luo Yingzi reveal the truth of the old case of Mei Daliang, and found that the original creator was Xu Zhuo. Xu Zhuo framed Chen Shuo into prison. At a critical moment, Luo Yingzi faced his feelings and became Chen Shuo's defender. Eventually, Xu Zhuo rope to justice. The relationship between Luo Yingzi and Chen Shuo finally took a step, and her career and Qiu Hua and Xia Shu's career also took the right track. Everyone once again completed their transformation and growth in the predicament."
"(Google translated): Luo Yingzi, Qiu Hua, and Xia Shu were swept out by the Liangcheng law firm. Luo Yingzi suspected that the incident was related to a old case she was checked by Mei Liang. In order to survive, the three started a law firm. In order to pursue Luo Yingzi's job change to the Liangcheng law firm, Chen Shuo fluttered. Luo Yingzi still did not accept Chen Shuo, but was fascinated by Xu Zhuo, a new partner. Chen Shuo secretly helped Luo Yingzi reveal the truth of the old case of Mei Daliang, and found that the original creator was Xu Zhuo. Xu Zhuo framed Chen Shuo into prison. At a critical moment, Luo Yingzi faced his feelings and became Chen Shuo's defender. Eventually, Xu Zhuo rope to justice. The relationship between Luo Yingzi and Chen Shuo finally took a step, and her career and Qiu Hua and Xia Shu's career also took the right track. Everyone once again completed their transformation and growth in the predicament."
So the new guy is the culprit of madam mei case.