Wireless sensor networks arranged in farmland give the farmland manager real time environment parameter data. The radio signal propagation environment changes a lot throughout the crop growth stage because of the changes of crop height, density and some other farmland environment parameters. This paper collected and analyzed radio signal propagation data of a complete growth period and established corn field path loss model for radio signal propagation to provide support to sensor node deployment and energy planning. The result shows that the radio signal propagation path loss exponent increases with the growth of corn; at the same corn growth stage, the radio signal propagation path loss exponent reduces according to the antenna height variation. This paper uses Lognormal distribution model to predict the radio signal propagation in corn field and the correlation coefficients between computational values and measurement values were between 0.906 and 0.998.