To quantitatively detect defects using reflective continuous-heat-excited thermography, a heat conduction model of an object under continuous heat excitation was established, and the temperature increment-time relationship on the thermal excitation surface of the object was derived. Based on an analysis of the temperature increment-time relationship on the thermal excitation surface, the depth of the defects could be measured by nonlinear fitting of the temperature increment-time data. To test the feasibility of this method, a GFRP flat-bottomed hole specimen was fabricated and analyzed using reflective continuous-heat-excited thermography. The results show that this method is highly accurate in measuring the depth of defects.