For better understanding the mechanisms of laser cleaning for contamination on surface of stonework, such as black ink and smoked dirt, the cleaning experiments are carried out. Its cleaning thresholds are obtained. Meanwhile, the finite element analysis software is used to calculate the temperature and stress of contamination under laser irradiation. The experimental results are discussed combining the calculation. The evaporation of contamination and thermal stress could be the main causes for removing of the pollutants. The site experiment of laser cleaning of black ink and smoked dirt in Yungang grottoes shows that this technique has a great advantage and can be applied for conservation of stonework.