The Shenzhen Stock Exchange started sampling 500 of 1,687 public companies on Wednesday for the Shenzhen Component Index, up from 40 firms previously.
"The aim of expanding the sample is to reflect stock market performance in a more accurate way," the bourse said in a statement.
The Shenzhen Component Index was launched in 1995, when just 120 companies were listed on the Shenzhen bourse, one of China's two stock exchanges. But 20 years on, the sample of 40 companies does not show a clear and comprehensive picture of the exchange's public firms.
The new Shenzhen Component Index will represent about 13 trillion yuan (2.1 trillion U.S. dollars) in market value, accounting for 61 percent of the total on the Shenzhen bourse, up from 18 percent.
After the expansion, the IT sector will have the largest share on the index in terms of market value.
The Shenzhen Component Index opened at 15,206.56 on Wednesday, up 0.52 percent.
Launched in 1991, the benchmark Shanghai Composite Index, another key stock indicator, tracks all 1,050 companies listed in Shanghai.
The sample expansion came amid bullish performance that has continued for nearly three quarters, and numerous small investors have been swarming to the stock market in search of quick wealth.
