How did the Communists, under Mao, win the peasant loyalty?

Like in almost every other popular communist uprising Mao believed that the peasants (read poor) were the most important. The doctrine he was following was that of Stalin and the USSR (although this changed slightly in later years). Basically the peasantry of China was dirt poor. They had always been dirt poor and under China at that time would always be dirt poor. There was no way to raise social status.

Mao simply told the peasantry that he felt they were important and that the rich were treating them as less than human. He promised them social status and promised to bring the rich down. In many ways he was not lying, the peasantry were treated very poorly by the rich and powerful in China. Mao did intend to bring the rich down and elevate peasants.

It was not at all difficult to gain popular support. Of course those supporters could not see what the future would bring in just a decade.