Based on the solar cell DC model and the mathematic expression of maximum power point,  the equations describing short circuit current,  open circuit voltage,  maximum power current,  voltage and filling factor varying along with the illumination intensity are deduced and calculated with two real solar cells data. The fact that short circuit current and maximum power current are proportional to the illumination intensity, and open circuit voltage and maximum power voltage are proportional to the natural logarithm of illumination intensity is certified. The relation of filling factor varying along with the illumination intensity is not a simple functional expression and effected by the solar cell model parameters is present. A simulated experiment is established to valid the theoretic analysis and calculating by Multisim.