The influences of cathode-mesh electric field on a streak image tube were studied in this paper. The results show that a critical field value following the pulse current density exists. The temporal resolution of the image tube can significantly be improved by increasing the electric field when the field strength less than the critical value, but if the field is greater, it lacks influence on the temporal resolution. Under the conditions of current density 20A/cm², the critical electric field is 10kV/mm. It is significance for designing the streak image tube.