The line must be as short as possible and away from the MOS tube drain line to prevent noise coupling, the signal should be arranged independently and separated from the power as far as possible. The optocoupler, Vcc and Y capacitors should be separated, and the feedback pin capacitance should be close to IC as possible.
Lay out the power supply parallel to the ground. Keep sensitive and high frequency wiring as far away from high disturbance power wiring as possible.
Widen the power line and ground line to reduce the impedance between the power line and ground line.
Minimize the loop area consisting of drain, clamp and transformer
Minimize the loop region consisting of secondary windings, output diodes, and output filter capacitors
Increase the distance between wires to reduce capacitive - coupled crosstalk.

2: Improper feedback design
For example, bandwidth setting is too wide, insufficient phase margin, the solution can try to pressure the bandwidth, some design to improve transient response, bandwidth too wide on the high frequency interference printing will be weakened, blindly increase the bandwidth is not desirable.