Abstract: Based on the Penman-Monteith Model(PMM), a method for calculating winter wheat transpiration in the field condition is established, and two main parameters involved in the PMM are anatysed. The emperical equations of net radiation intercepted by canopy (R_n^') and total canopy leaf resistance (r_8T) are presented, and their precision is checked. The relation between the actual transpiration in the deficient water condition and potential transpiration in the safficient water condition is analysed.