In high performance parallel computing, energy optimization for parallel loops becomes one key because the time of loops often takes a significant part of the whole execution time. Energy-constrained problem is one of the important research focuses. This paper studies energy-constrained problem based on OpenMP static loop scheduling. Firstly, we propose energy-constrained static scheduling algorithm (ECSS), which utilizes DVS to scale down voltage/frequency of the light-loaded processors in terms of energy constraint. Secondly, we propose Energy Constraint based Performance-Optimal Static Scheduling algorithm (ECPOSS), which combines loop rescheduling and DVS for the better performance under the same energy constraint. We prove ECPOSS can obtain the best performance under the same energy constraint. Through testing NPB3.2-OMP programs on 20-160 multiprocessor simulation environment, we evaluate the effectiveness of our algorithms. Experimental results show the performance of ECPOSS is better than that of ECSS by 4.81% under the 50% energy constraint on 100 processors.