Abstract: Most of local deep convections are initiated in a conditionally unstable environment. Thereby it is necessary to consider the influence of a deep air layer to the air parcel that rises from the base level. Some questions related to conditional instability, such as the unstable energy and Convective Available Potential Energy (CAPE), are discussed from the perspective of energy. In addition, the relationship between the moist static energy of a single surface station and the unstable energy in a conditionally unstable environment is analyzed based on a case study using observational data. The main results are as follows. (1) From the perspective of energy, the original definition of CAPE should be followed. That means CAPE is equal to the difference between positive and negative unstable energy. The lifting velocity in the lower level associated with the kinetic energy is implicitly included in the formulation of original CAPE, which is helpful for forecasting severe convective weather, especially in South China in the warm season. (2) The moist static energy remains constant when the air parcel is lifted from low to high levels during both the dry adiabatic and moist adiabatic processes, which indicates that the variation of total energy is only related to the variation of kinetic energy caused by external forcing. There exists no transformation from moist static energy to kinetic energy. (3) Temporal variation of the moist static energy at a single surface station indicates the variation of positive and negative unstable energy, which is closely related to the calculation of unstable energy. Such temporal variations at various stations can make up for the weaknesses caused by coarse resolution of soundings and the deficiency of hourly data of positive and negative unstable energy.