In autumn, the weak convection case covers more percentage in Huabei (North China). In order to improve the ability to diagnose the convection intensity, two linear cases are analyzed which develop in the similar synoptic background but in different strength. The result indicates that strong convection develops in the deep upward current (to 200 hPa). And the absolute humidity in the lower troposphere is higher. However, in the weak one, the ascending current exists below 500 hPa only. At the same time, the specific humidity in the BL (boundary layer) is just (5-7) g·kg-1 which is (2-5) g·kg-1 lower than that of the strong case. These lead to the convection in different strength mainly. The deep anaflow in the strong case is caused by convergence line, dewpoint front and upper level trough. Furthermore, a strong divergence appearing in the upper troposphere overlaps the convergence layer. As the result, upward current enhances and goes up further. But for weak case, the cold air invades to the mid troposphere and makes the ascending current which could reach to the upper levels. The thermodynamic background difference appears in the lower troposphere. When strong convection develops, the instable energy accumulates in the convective area and CAPE reaches 1087 J·kg-1. Besides, from the surface to 600 or 700 hPa, it is warm and moist and being instable also. In the weak case, CAPE is 68 J·kg-1. Above the BL, it is cold and dry comparatively. Therefore, the following items could be helpful in the diagnosis of the convection strength in autumn: the depth of the upward current, the absolute humidity in the lower troposphere, the thickness of a layer which is not only unstable but also warm and wet.