To describe charge properties of space dusty plasma, we applied the detailed balance method which takes adequately into account the discreteness of charge inflow on a dust grain. A system of equations is obtained which enables us to find analytically the shape of the charge probabilities with great and small charges on a dusty grain. We found the conditions under which the steady-state charge distribution is Gaussian shaped one. The grain surface potential is demonstrated to depends on the grain radius but to be not constant as generally believed.