The authorities are worried about the possible economic effects and have already tried preventative measures.Twenty-one brokerages agreed the move at a meeting in Beijing on Saturday, Xinhua news agency said.
The brokerages "will jointly invest 15% of net assets as of the end of June, or no less than 120bn yuan, in blue chip exchange traded funds", the Securities Association of China said in a statement.
★net assets：净资产。net 除了网，在和钱连用时多表示净，比如 net profit净利润。和它相对应的单词是gross，又比如gross interest就是毛利率。
The brokerages have also committed themselves not to sell off holdings while the Shanghai Composite is below 4,500.
The statement also expressed "full confidence" in the development of China's capital markets and urged all brokerages "to view the economic situation and capital market in a correct way and take similar actions to underpin（支持，巩固） the ailing market".
The Chinese stock market bubble over the past year has been fuelled in part by investors borrowing heavily.Now that shares are on a downward path, policy makers are concerned the wider economy could be affected.
The property market has been beset with problems and local authorities are burdened with high levels of debt.The authorities have sought to halt the share slide with an interest rate cut and by injecting more cash into the economy.
★beset sb with / by sth：困扰，威胁 The team was beset by injury all season. 球队整个赛季都因队员受伤而受困扰。
★be burdened with sth.被…所困扰。
Last week, China's securities regulator （证卷监管者）announced an investigation into possible manipulation of the stock market.