The status of PM_2.5 concentration in the urban forest can directly reflect the function of urban forest reducing PM_2.5 pollution, and it is also the important environmental issue that residents concern about during recreating in forest. PM_2.5 concentrations in three recreation urban forests were simultaneously measured in typical days over the four seasons in the West Mountain of Beijing, China. We analyzed the seasonal and diurnal variation of PM_2.5 concentrations in three different recreation urban forests and their influencing factors in this study. The results were as follows: (1) PM_2.5 concentrations in three recreation forests in the West Mountain were much lower than that in the urban background of Beijing in most of the time over the year. PM_2.5 concentrations in three recreation forests achieved the national standard for urban areas in spring, summer and autumn. Furthermore, the concentration was even as low as the national standard set for the first class region in spring and autumn. (2) PM_2.5 concentrations changed in the order of winter >summer>autumn>spring, indicating that the purifying effect of urban forests on PM_2.5 varied in different seasons. (3) PM_2.5 concentrations in forest changed greatly in 24 hours a day. In general the concentrations during night time were higher than that of the day time, and diurnal variation curves showed nearly "two peaks and two vales", with two peaks appearing at night and in the morning, and two troughs in the early morning and around noon. Throughout the year, vales appeared in the day time at around 15:00 in spring, 13:00-17:00 in summer, 13:00-15:00 in autumn, 9:00-11:00 in winter. (4) The trend and the values of PM_2.5 concentrations were different from each other in three kinds of urban forests. In Platycladus orientalis forest with higher canopy density, the peak time and vale time of PM_2.5 concentrations at night occurred later, PM_2.5 concentration was higher at peaks time, and the duration of the peak was longer than that in other two forests. Therefore, PM_2.5 concentration at night in Platycladus orientalis forest was higher than that in Cotinus coggygria forest and the mixed forest, especially in autumn. We, therefore, concluded that the urban forests in the West Mountain of Beijing can provide a relatively healthy forest recreation environment for the residents in the urban area. Spring, autumn and summer are the comparatively fine seasons for Beijing residents to recreate in the West Mountain, and the healthy time according to the status of PM_2.5 concentration are the whole day in spring, 9:00-17:00 in summer, the whole day in autumn and 9:00-11:00 in winter.