The invention discloses a method for determining related substances in avanafil and a preparation thereof. The method is characterized in that a chromatographic column taking octyl bonded silica gel as a filler is adopted, phosphate buffer solution-acetonitrile is taken as a mobile phase, gradient elution is performed, and a diode array detector or an ultraviolet detector is adopted. The analysismethod is advantaged in that the avanafil and nine process impurities thereof can be separated, degraded impurities can further be effectively separated, and a blank solvent does not interfere with measurement, common instruments, equipment and the chromatographic column are used, the mobile phase is simple and easy to obtain, operation is simple and convenient, sensitivity is high, and the quality of avanafil and the preparation thereof can be better controlled.