The effect of null space of projecting data on reconstruction result is discussed. Then the convergence procedure of Algebra Reconstruction Technique (ART) and its reconstruction result for incomplete projecting data are analyzed. Simulated Annealing(SA) method in statistic physics is introduced into the iteration of ART method to deal with the problem of incomplete constraint. Based on the assumption of continuity among image pixels

constraint of most stable solution is added to get the optimal solution by means of Simulated Annealing method.