Gender differences in achievement in mathematics, a traditionally male-stereotyped subject, have long been a concern for many educators around the world. Gender differences in mathematical achievement have decreased in recent decades, especially in Western countries, and become small or insignificant in large-scale tests, such as the Programme for International Student Assessment (PISA). The situation in China has not yet been studied. The recent PISA report lists China B-S-J-G (representing Beijing--Shanghai--Jiangsu--Guangdong) as an educational system with no significant gender difference in mathematical achievement. Based on a secondary analysis of PISA 2015 mathematics data from China B-S-J-G, this study more deeply scrutinized gender differences in Chinese students' mathematical performance, emphasizing societal factors, namely students' socioeconomic status, school level, school type, school location, and socioeconomic status at school level. This analysis revealed important differences within the overall picture. Most importantly, significantly more boys than girls scored in the top tier of mathematics achievement. At the lower- and upper-secondary school levels, boys performed significantly better than girls, with the achievement difference increasing at the upper-secondary level. Furthermore, this study found that, on average, Chinese (B-S-J-G) girls achieved significantly lower average scores on the PISA 2015 mathematics test than boys in the same school. Overall, students' individual characteristics and school characteristics need to be separated and both taken into account to examine the role of gender in mathematical achievement, which has not been thoroughly investigated in the past.