When the shape of single wood spheres was captured and processed under different pyrolysis environments instantaneously, the evolution of sample geometry was obtained and a shrinkage model was proposed in this paper. The proposed shrinkage model was coupled with a one-dimensional unsteady wood pyrolysis model to predict the temperature profiles and mass variation as well as product distribution within wood spheres. A one-step drying mechanism and three parallel primary decomposition reactions as well as three secondary cracking reactions were used to describe the entire pyrolysis process. Experiments were carried out to assess the efficiency of the model prediction, and the effects of volume shrinkage on the temperature, weight loss, and product distribution were analyzed and discussed. Experimental results show that the shrinkage rate is proportional to the furnace temperature, and the average shrinkage rate increases from 0.39 to 0.53 mm/min for 20 mm wood spheres and from 0.26 to 0.55 mm/min for 30 mm wood spheres when the furnace temperature increases from 673 to 973 K. However, the final shrinkage ratio is inversely proportional to the furnace temperature and decreases with the increased sphere diameter. The peaks of simulated temperature profiles are in good agreement with experimental results when shrinkage is considered. Simulated mass loss profiles with shrinkage agree well with experimental data. On the contrary, if a constant particle size is used, the deviation between simulated and measured residual fractions is about 24% for the wood spheres studied in this paper.