Actual evapotranspiration and components of field energy balance were measured by eddy correlation system, and the variations of Priestley-Taylor parameter (α), characterizing the energy partition over winter wheat field surface were analyzed. At the diurnal course, daily average of parameter α was 1.324±0.334, at noon, the value was 1.05-1.17. And the graph of parameter α with time had a concave-up shape from 7︰00 to 18︰00 after regreening stage in winter wheat field in the North China Plain. The seasonal average of parameter α was 1.473±0.454, above the value of 1.26 under wetness surface. The maximum of parameter α was 2.317 under the condition of enough soil water content during winter wheat tassel and anthesis stage, and the minimum of parameter α was 0.410 with soil water stress during stem elongation in winter wheat field. Water use efficiency of small area irrigation in wheat winter field is decreased because of heat advection under drought condition.