One typical structure of typhoon is the outflow layer in the upper troposphere. However, there were relatively few studies on the outflow layer. In terms of criterion Richardson number, the data from numerical simulations of the non-hydrostatic mesoscale model WRF on typhoon Chanchu (2006) was used to investigate low Richardson number in the upper troposphere, as well as the structure and causes of the low Richardson layer. Results demonstrate that the prominent frequency of low Richardson number that even reached the turbulence criticality existed in the altitudes ranging from 10 km to 16 km, supporting the assumption of Richardson number criticality in tropical cyclone by Emanuel (2011). By comparing differences in the low Richardson number layer during the newly-born stage, developing stage, mature stage and decaying stage of the tropical cyclone, a correlation was found between the frequency of low Richardson number and the strength of the tropical cyclone, namely, the low Richardson number layer was found to be more common and broader in the stronger phase of the tropical cyclone. The presence and distribution of low Richardson number in the outflow layer was characterized by vertical wind shear and static stability. Along with the process of slowly changing of typhoon intensity, the frequency and distribution of low Richardson number exhibited a diurnal variation, which was associated with the change in static stability caused by diurnal variation of solar radiation. This study revealed the existence of upper-tropospheric low Richardson layer in tropical cyclones, and verified the sensitivity of Richardson number to cyclone intensity and the radiation diurnal cycle.