The primary-side regulation (PSR) flyback converters with the feature of constant current (CC) and constant voltage (CV) are widely used in charger systems and adaptors of electronic devices. However, the conventional method of CC control is complex and inaccuracy, and more circuits should be added to improve accuracy causing more complex design and larger cost. In this paper, a novel strategy of CC control strategy based on large signal control is proposed to achieve simple and high-precision CC control which operates seamlessly as working state switched between CC and CV. The control chip is implemented in a 0.18μm BCD process, and the deviation of output current in CC mode is less than ±2% without loop compensation. It well meets the demand of large charging current in startup and limiting current under low voltage condition.