On the basis of recognition of the geology characteristics of the Nihe iron deposit, this study tries to use cutoff grade to delineation ore bodies with single grade index and use Spline curves to smooth the interpretation boundaries of each section, and the gradient mineralized domain has been established. For the gradient mineralized domain used, the unreasonable intersection among complex shape of ore bodies which result from delineation ore bodies with double grade index has been eliminated, the sample assay data used for the reserves estimation has better continuity and comprehensiveness, and the grade distribution and the reserves can be estimated more reasonable and efficiently.