Crab apples need very little pruning. In fact, they are best left unpruned.

You should, however, remove water sprouts and suckers. Water sprouts are thin shoots that grow vertically from branches. Cut them back close to the branch. Suckers grow up from the roots of the tree (usually from the stock on which the tree was grafted). Cut them back to just below the soil surface.

Dead branches should also be removed. Any other pruning is best done when the tree is dormant, e.g. late winter. If you want to prune to shape the tree or if it is too large for the location, then do a minimal amount of trimming. Make any cuts close to a branch or bud. It is helpful to remove any inward growing branches or branches that cross each other to open up the structure of the tree.

Courtesy of NYBG Plant Information