My dog beats up my other dog?

I dont know why, but my sharpai keeps on tackling and chewing on my other dogs head. My other dog is a bull mastiff/pit bull.. how do I stop this? I feel sorry for my dog when I open the door, and my dog comes bolting through and locking his jaw onto my other dogs face. Dog 1(mean one) only makes growly noises and it sounds like their playing but dog 2(the nice one) gets beat so harsh.