Objective: To explore the exercise intensity and effective exercise load of different physical fitness classes for second year junior high school students in Beijing, so as to provide a reference for optimizing the teaching content of physical fitness classes, reasonably regulating exercise load and the scientific development of physical fitness. Methods: From October to November 2022, 30 second year junior high school students from a middle school in Xicheng District, Beijing were selected to have their heart rate monitored during physical fitness classes using Polar heart rate monitors. Heart rate intervals and training impulse (TRIMP) were used to quantify exercise intensity and load. Independent sample t-test was used to statistically analyze differences in heart rate across different dimensions. Results: The average heart rate of students in physical fitness classes was (140.62±9.41) counts/min, with effective exercise load (heart rates ≥120 counts/min) time accounting for 77.2%, and heart rate load of moderate to vigorous physical activity (MVPA) time accounting for 51.9%. The average heart rates for speed, speed endurance, and endurance physical fitness classes were (137.89±8.82) (137.67±11.27) and (145.35±8.98) counts/min, respectively. The average heart rates of male and female students in physical education class were (144.22±24.95) and (136.31±28.78) counts/min, and the difference was not statistically significant ( t=4.04, P >0.05). The cumulative durations of effective exercise load among male and female students were (34.08± 7.52 ) and ( 28.43 ±5.39) min, respectively. The average TRIMP value for physical fitness classes were (100.46±16.56), with a exercise density of 72.06%. Conclusions The exercise intensity of physical fitness classes for secondyear junior high school students in Beijing is appropriate, mainly with moderate to vigorous intensity, and the accumulated time of MVPA is sufficient. The average heart rate and exercise density of students meet the requirements. Scientific setting of physical fitness class exercise load should be based on individual differences in students physical fitness, in order to improve the physical health level of middle school students.