I found a surge protection circuit.
If we don't take any measure, there will be a huge surge caused by the charge of C4. So I think the working principle is like this: C6 is gradually charged until VSG reachs the open threshold value of PMOS. Then Q4 is gradually opened so that there won't be large surge.
So my expected wavefrom of current IL is like this:
But when I simulated the circuit, the wavefrom of IL is as fellow:
The current is zero. There is no charge process of C4, although the MOS Q4 is opened as VSG is 17.7, greater than the threshold value of Q4. I don't know why.