Taiwan remains a particular area of focus. The Chinese army upgraded many of the more than 1,000 short-range missiles aimed across the Taiwan Strait, the report found.
"China continues to deploy many of its most advanced systems to the military regions opposite Taiwan," it said.
But China has also expanded its military operations beyond its immediate territory, like carrying out anti-piracy missions in the Gulf of Aden.
The study also said that China has an active programme to build aircraft carriers, and that construction on the first one could begin later this year. It also reported that China plans to expand its fleet of nuclear-powered submarines.
The report concluded that "many uncertainties remain regarding how China will use its expanding military capabilities".
Ike Skelton, the chairman of the armed services committee in the US house of representatives, called for greater transparency from the Chinese army.
The lack of communication raises "a dangerous risk that miscommunication and misperception between the US and China could lead to a miscalculation," Skelton said in a statement.
China has not yet commented on the report. But Beijing sharply criticised a similar report last year, saying it had a "baneful influence" on ties between the US and China.
Diplomatic ties between the two countries have been strained for several months: China froze military relations with the US in January after Washington announced a sale of helicopters, missiles and other military gear to Taiwan.
Despite the rapid growth, China's military spending remains just a small fraction of the US defence budget, which this year topped $660bn.