In this paper, the SEIR model is generalized by introducing an asymptomatic class to quantify the influence of wearing N95 facemasks in reducing the spread of influenza H1N1. What´s more, we explain the control reproduction number Rc according to the biological meaning reasonably. Without any intervention, the basic reproduction number R0=1.83 and there will be a large outbreak of infectious diseases. If N95 facemasks are 50% effective in reducing susceptibility and infectivity, 50% of population wear them on the first day, the basic reproduction number will be decreased from 1.83 to 1.17 and the final size reduced from 73% to 2%.