The intrinsic attribute of an object is thermal inertia, which consists of unchangeable elements such as thermal conductivity, density, and specific heat capacity. By absorbing and emitting solar energy, the temperature of details increases in the daytime and decreases in the night. Details of high thermal inertia, such as water, show small temperature difference between day and night. On the contrary, details with low thermal inertia, warming fast in daytime and cooling fast at night, show substantial temperature discrepancy between day and night, such as bare rock. Therefore, details of different thermal inertia in the temperature difference map between day and night can be distinguished.17,18 The typical details in karst area, besides built-up land, water, and road (mainly refers to cement and asphalt roads which can easily be mixed up with bare rock road), can be divided into three categories: vegetation, soil, and rock,19,20 among which built-up land and roads have been removed by masking. The vegetation is with large thermal inertia because vegetation contains a larger quantity of water, which has the largest thermal inertia; the thermal inertia of soil is different because of the different soil moisture content; and the thermal inertia of bare rock and bare soil with little moisture is the smallest. This paper studies the genesis of abnormal heat island formation with temperature difference maps between day and night generated by MODIS LST of the corresponding phase.