With the help of the floating sphere mirror image model and the Fowler-Nordheim equation, the emission current density from carbon nanotube was calculated in this paper. Taking the field enhancement factor and the emission current density into account, the field emission of carbon nanotube array was optimized in theory. The result shoved that the field emission from carbon nanotube array is maximal when the intertube distance is equal to one tenth of the tube height, which is a little smaller than the previous result. When the intertube distance is larger , the field enhancement factor increases, but the emission current density decreases even more rapidly with increasing intertube distance. For carbon nanotube arrays with higher array density, the field enhancement factor decreases greatly with the decreasing intertube distance due to the screening of the electric field.