Abstract: Based on a laser calorimetry instrument, the error source of the measurement or uncertainty of weak absorption were analyzed with the exponential method, pulse method and gradient method given in ISO 11551. The uncertainties of the parameters such as mass and laser power were evaluated by B method, while the uncertainties of the parameters induced by fitting and residual errors were evaluated by regression analysis and calculated by MATLAB. A sample was picked up randomly to make measurement repetitively, proving the feasibility of the evaluation. It was revealed that the fitting error contributed notably to the uncertainty of absorption calculation. The relative uncertainty of exponential method was approximately 0.0129, and the uncertainty of the pulse method was 0.0029, proving that the pulse method was the best among the three. The gradient method adopted no more than 2 points in temperature data, and had a relative uncertainty about 0.0961. The promising way to increase the precision of calorimetry is to improve the data processing method and the laser power measurement.