To get the explosion rule of field ammunition detonated by jet, the explosive with various spaced targets was used as field ammunition simplified model. The jet initiation models of shield explosive were established and calculated by impedance match technology, theory of penetration by quasi-steady fluid dynamics and Held criterion. Results show that the biggest detonating abilities of shock wave and remainder jet are about 2.75×10⁴ m³·s^-2 and 1.55×10⁵ m³·s^-2, respectively, which prove that the primary influence factor of detonating ability is the bomb case comparing with protective coating. During the field ammunition detonated by jet, the changing of jet diameter would be to 22.2% and its influence could not be ignored. Furthermore, preventing the penetration of jet is the first focus of field ammunition, for the detonating ability of remainder jet is more than the shock wave.