A sauna weather event in northern China in July 2009 is numerically simulated and diagnostically analyzed. The atmospheric circulation characteristics, the horizontal and vertical distributions of temperatures and moistures, and the distribution of potential vorticities are studied. It is found that anticyclone dominates the upper troposphere during the sauna weather event. In the horizontal chart, the relative humidity in the lower troposphere is large. From the vertical sections, descending airflow dominates the moist and warm sectors at the middle and lower levels. There are clear humidity gradient, evident vertical gradient of temperature, and slanting distribution of potential vorticity. The sauna weather event occurs in the generally high-temperature environment of summertime, so it is difficult to dynamically identify and diagnose the sauna weather depending on some single factor, e.g., temperature or moisture. According to the high-temperature, large-moisture, and strong-potential vorticity characteristics during the sauna weather in northern China, a moist thermal potential vorticity parameter MTPV, may be expressed as ▽ q (▽ ▽ Q), where q is the sum of water vapor and all hydrometeors including cloud water, rain water, cloud ice, snow and graupel; is potential temperature; Q is potential vorticity) which is appropriate for sauna weather, is introduced. Then it is used to dynamically diagnose sauna weather event. And it is simplified by calculation analysis in case study. It is found that the MTPV anomaly is accompanied by the sauna weather process. Although high temperature, large moisture and strong slanting potential vorticity development present in the sauna weather process in northern China in July 2009, their coverages for these single variables are larger than our target region of this sauna weather event. While for the MTPV and its simplified form combining these variables, their anomalies maintain in the lower troposphere around Beijing and its peripheral areas in northern China, viewing from either zonal or meridional section. Therefore, both MTPV and its simplified form can better dynamically identify this high-temperature and high-humidity sauna weather event.