SHANGHAI—Long derided as a casino, China’s once-volatile stock market is going through a long stretch of calm. One reason is an orchestrated government effort to keep traders and investors in line.

Three years after a national uproar when Chinese stocks plunged by nearly half in just over two months, traders and brokers say regulators are increasingly stepping in to influence trades and make China’s markets appear less volatile, especially during political events when Beijing wants to project stability.