we are a registered non-profit society. We need to open a corporate account for our Association. However, we don't have sufficient fund to maintain the minimum deposit of $10,000 for a corporate account.

one suggestion is that the president and vice presidents of society loan enough money to the society to open a corporate bank account. according this suggestion, this loan will be RETURNED when the president/vice president resign from the official position or when the society cumulate enough fund (S$10,000)as deposit, which is earlier.
This fund will NOT be spent, and will stay on society's balance sheet, as cash at bank, and also non-interest-to individual loaner.

one of our CONSTITUTION is that
The Society shall not raise funds from the public for whatever purposes without the prior approval in writing of the Assistant Director Operations, Licensing Division, Singapore Police Force and other relevant authorities.

In this case, my question is:
Is it legal that the society can take loan from it's president and vice presidents to open a corporate bank account?

If your association’s Constitution does not prohibit raising fund from his office bearers then it is not illegal but it is recommended you have an agreement between the society and the office bearers to put this or record.