We use the Hierarchical Linear Model to analyze the current state of extracurricular tutoring time for secondary school students and its impact on academic performance using PISA2015 data from four provincial administrative regions in China. The study discovered that the effect of extracurricular tutoring time on academic performance is a non-linear relationship that initially declines and then increases, indicating that subject-specific tutoring have a threshold effect. When tutoring time exceeds a certain threshold, academic performance improves qualitatively. However, when mathematics tutoring time exceeds a certain threshold, the gap in mathematics scores between students from different family backgrounds widens, resulting in educational inequality. The effect of school curriculum time on student achievement is a non-linear relationship that begins with an increase and then declines. When compared to off-campus tutoring time, in-school curriculum time can more effectively improve student academic performance.