Abstract: At present in practice of hog cholera preventing and controlling ,the effect of ultra pre-immunity to antibody level change and protection still existence dispute in the four latter nest piglets.Four nests Duroc×Landrace×Large White piglets with similar breed ,age and fetus frequency were selected ,five piglets of each nest were used to test ultra pre—immunity and pick venous blood in the age of 20,30,40,and 60 days , hog cholera detection kit of IDEXX company were also used to detect blocking rate of hog cholera.The result showed that after ultra pre—immunity , strengthen another hog cholera vaccine in the age of 50 days can perform a good protection to piglets.