After accounting for a contact resistance between carbon nanotube (CNT) and substrate, the local electric filed of top of CNT were calculated by combining an improved floated sphere model and the Fowler-Nordheim theory for understand the effect of the contact resistance on field emission from CNT. It is found that the field emission current is limited and current saturation and nonlinear characteristics of the Fowler-Nordheim plots are produced by the contact resistance in region of high electric field. The origin can be attributed to the local electric filed of top of CNT remarkably debases as compared to without the contact resistance.