This study established a K drought monitoring model to determinate the start, duration, and strength of drought. Based on 586 meteorological stations’ conventional observation data in China from 1961 to 2015, we calculated the elements and parameters needed to structure the model. Because soil has a memory of its antecedent moisture change, the drought intensity of a day is determined by the moisture content of the day and earlier days. Furthermore, the change of moisture content depends not only on precipitation but also on the temperature associated with evaporation. In order to consider the impact of previous precipitation and evaporation on the present drought, we made a cumulative treatment of previous precipitation and reference evapotranspiration. However, the influence degree of the antecedent precipitation and reference evapotranspiration on soil moisture is weakened closer to the present day. This means that the contribution of the precipitation and reference evapotranspiration in earlier days to present soil moisture decays over time. After comparison between the exponential decay form and the linear decay form of the previous precipitation and reference evapotranspiration, the scheme of linear decay form is selected. Then, a K drought monitoring model was established to consider the antecedent precipitation and reference evapotranspiration accumulation and their decay influence. The proposed K drought monitoring model can be used to evaluate the timing of drought onset, evolution process, and severity.