A large cathode magnetic-focusing framing image converter tube is designed and developed. The field curvatures of single magnetic lens and double magnetic lens systems are investigated at electron optical magnification of 2∶1. Based on optical lens imaging principle, the approximate relational expressions of the imaging excitation variation with the best image positions of the off-axis object are derived in the cases of single magnetic lens and separated double magnetic lens systems. It is assumed that the shape of image surface does not change with excitation in a certain scope. Accordingly, a method of field curvature measurement and decrease is proposed. Then, by virtue of Matlab programming, the best image positions for off-axis point objects are simulated, and the image surface equations are fitted, which are experimentally verified by use of the proposed method of field curvature measurement. It is showed that computer simulations agree with the experimental results within a certain field of view. Both of the best image surfaces of two systems are paraboloids of revolution. But the field curvature of double magnetic lens system is obviously smaller than that of single magnetic lens system.