There have been lots of methods on the prediction of grain yield,however these predicted results could not be very accurate due to all kinds of factors that affect on grain yield. In this article,three different kinds of theoretic probability methods (i.e,the normal distribution curve probability,converting the abnormal distribution curve to normal curve,the integration of analytical probability density curve are used to calculate the probability of risk of any given grain yield sequence. These methods showed a wide range of increase and decrease of the crop yield as well as the corresponding high and low probability,which can provide good references to the development of production,natural disaster prevention and alleviation.