With the development of cloud computing, more and more mobile devices have the requirement to outsource expensive computations in an untrusted environment. Among all such computations, exponentiations modulo a large prime are basic and frequent operations in many discrete-logarithm-based cryptographic protocols. Currently the most efficient outsourcing algorithm of modular exponentiations is under a one-malicious version of a two-untrusted-program assumption. And the result of the algorithm is checkable with a probability at most 2/3. We here propose an efficient algorithm under a two-untrusted- program assumption and improve the probability of checkability to be about 1-2/3s for a suitable s configured by a client. Further, we provide practical results to show the energy saving about outsourcing modular exponentiations.