Statistics. Most econometrics people are major in Statistics. By the way, most statistical methods used in economics as a research tool are misused because they do not understand what the real statistics is. If you are good at statistics, you will find out this when you read economic paper.

then what do you think the real statistics is and how to learn it well

数学的话比数学专业的少一点------数学分析，常微分，高等代数，实变函数与泛函分析，概率论与数理统计