Abstract: In this paper it is deduced by theory and imitated by the curve of experiment data that the model of thickness change by absorbing water for wheat straw particleboard can be expressed with " TS(t)=a-be ". It can reflect its dimensional stability in certain degree. In this equation "a" de- notes the maximum value of thickness swell for a wheat straw particleboard and " " expresses a pa- rameter which reflects the size stability of wheat straw particleboard. The bigger its value is, the lon- ger the time of reaching the maximum value of thickness swell for a wheat straw particleboard and the better in a certain time its size stability is. The result of the model application shows that the more the dosage of MDI is,the smaller the maximum value of thickness swell for the wheat straw particleboard is,the bigger the coefficient is, the higher the dimensional stability is. When the density of wheat straw particleboard increases from 0. 65 g/cm3 to 0. 75 g/cm3 ,its dimensional stability is improved a lit- tle. When the time of hot-pressing increases from 6 min to 10 min for the wheat straw particleboard of 10 mm thickness and the temperature of hot-pressing 150℃,the maximum value of thickness swell for the wheat straw particleboard first decreases and then increases,the coefficient T first increases and then decreases.