ETF fund is a transactional open index fund, also known as ExchangeTradedFunds (ETF), which is an open fund with variable fund share. Transactional open-end index fund is a special type of open-end fund, which combines the operating characteristics of closed-end fund and open-end fund. Investors can buy or redeem fund shares from fund management companies, and at the same time, they can buy and sell ETF shares in the secondary market at the market price like closed-end funds. However, the purchase and redemption must use a basket of shares for fund shares or use a basket of shares for fund shares. Because the securities market transaction and the subscription and redemption mechanism exist at the same time, when there is a price difference between the ETF market price and the net value of the fund unit, investors can carry out arbitrage trading. The existence of arbitrage mechanism makes ETF avoid the common discount problem of closed-end funds.