  Based on the method of collective variables we develop the statistical field
theory for the study of a simple charge-asymmetric $1:z$ primitive model (SPM).
It is shown that the well-known approximations for the free energy, in
particular DHLL and ORPA, can be obtained within the framework of this theory.
In order to study the gas-liquid critical point of SPM we propose the method
for the calculation of chemical potential conjugate to the total number density
which allows us to take into account the higher order fluctuation effects. As a
result, the gas-liquid phase diagrams are calculated for $z=2-4$. The results
demonstrate the qualitative agreement with MC simulation data: critical
temperature decreases when $z$ increases and critical density increases rapidly
with $z$.
