The invention discloses an onsite thermal property testing method under the condition of a non-constant power. The trend of a heating power or cooling power changing with time is analyzed under the condition of power instability such as the interruption of a power supply and large fluctuation of voltage, and by dividing the non-constant power into a plurality of sections in terms of time, wherein, the power of each section is comparatively constant (the power can be considered as zero when heating or cooling are interrupted); the heat-transfer process of buried pipes is considered to undergo a plurality of step heating flux actions which are further disintegrated into a superposition of a plurality of thermal pulses; linear superposition principle is adopted for analyzing and a mathematical model is established; and ground thermal property parameters can be obtained by integrating the mathematical model with a parameter estimation method and other methods.