Not illegal. If you are a qualified investor with investment qualifications, it is usually not illegal to open a sub-account and buy CSI 500 securities investment fund products under a private equity institution established and filed according to law and arranged for an appropriate investor range, even if it is opened online. However, it should be noted that before opening a sub-account and trading, you should understand the risk characteristics, investment strategies, costs and other related information of private equity products, and invest according to your own financial situation and risk preference to avoid the financial risks brought by blind personal decision-making.