Low pressure axial flow fans are usually installed in places where people work, amuse themselves and rest. The requirement for low noise is especially important. High efficiency, compact structure, low price and attractive external form are required. The flow type is one of the important aerodynamic parameters in the design of axial flow fans. According to the noise creation mechanism of low pressure axial flow fans and boundary layer theory, the optimal design for a cascade flow type fan was investigated. The investigation was designed to decrease aerodynamic noise and increase efficiency by means of optimization theory.