An under-gate field emission display (FED) with a concave cathode is proposed. Ansys software is used to simulate the electric field distribution and the focus effect of electron beams. The curvature radius of a concave cathode which affects the self-focus is investigated. The results demonstrate that the concave cathode focuses electron beams which emit from cathode obviously. As the curvature radius of a concave cathode decreases, the focus effect is enhanced and the radius of spots on the anode decreases too. A 10 cm×10 cm FED with a good self-focus effect is successfully prepared using screen printing. The field emission image shows that the structure can decrease the size of spots and improve the resolution. The anode current is well modulated by a gate voltage of 100～250 V under an applied anode voltage of 600 V.