Abstract: When a circular foil heat flux sensor is used to measure aerodynamic heating in a hypersonic convection environment, the surface temperature of the sensor is often lower than the surface temperature of the measured object. This surface temperature discontinuity would affect the flow of the boundary layer, which could distort the heat flux measurement result. For a blunt-head plate model, the numerical simulation method was used to study the formation mechanism of the "cold-spot effect" and the influence of the local low temperature of the sensor surface on the surface heat flux under hypersonic flow conditions. The results show that:the higher H_w/H_re is, which represents the ratio of the surface enthalpy of the measured object to the recovery enthalpy of the flow, the more obvious the "cold-spot effect" is; the lower T_w2/T_w1 is, which represents the ratio of the sensor surface temperature to the measured surface temperature, the more obvious the "cold-spot effect" is; the flow Reynolds number Re has less effect on the cold spot effect. The deviation of measurement results in response to the "cold spot effect" is analyzed under the condition of Mach 18 flow. The results show that the "cold spot effect" can make the measurement result 1.25 times higher, which reproduces the difference between the heat flow prediction results and the test results.