Abstract: Focus on an efficient and simple gear-design method for external gear pumps, from no undercut, standardization of addendum coefficient and tip pressure angle, pump-performance parameters was derived, and then from working-condition parameters to pump-performance parameters to tooth-profile parameters to gear-design parameters, a new reverse gear-design method was constructed. All results showed that tooth-number was an leading tooth-profile parameter, the minimum tooth-number was six, and the reverse gear-design method was simple and efficient; contact-ratio was a direct factor affecting flow ripple, the more tooth-number was, the greater contact-ratio was, the better transmission stability and flow ripple quality were, but the worse lightweight effect and trapped-oil effect were, etc.