A nanosecond-risetime HV（high-voltage） EFT（electrical fast transient/burst） generator which can simulate VFTO（very fast transient over-voltage） is demanded for secondary equipments’ EFT immunity testing. According to IEC 61000-4-4/GB 17626.4—1998， PSPICE is used to simulate the circuit parameters. Solving differential equations in LAPLACE transformation with MATLAB validates the acquired parameters. A HV EFT generator is developed， which adopts switching mode power supply as HV DC power supply and hydrogen thyratron as switch. It has such advantages as small volume， lightness， efficiency and cheapness. Experimental results prove that the EFT generator meets the requirements of EFT immunity testing.