The noise factor of the electron multiplying CCD was derived based on Poisson distribution model whose theoretical limit was 2, the same as binomial model. The input and output characteristics of the electron multiplying register and the additional noise introduced by signal multiplication process were quantitatively described. Considering the defects of coating test, the image information of uniform illuminated was analyzed and processed. The influence of readout noise, fixed pattern noise and background values to test results was eliminated. This method reduced the test error and improved the precision. Meanwhile, the chip was uncoated and test process was simplified. Based on theoretical study, the experiments were carried out to test Andor Luca camera. The results show that when the gain is greater than 100, the electron multiplying CCD noise factor is 1.414, consistent with the theoretical values. This demonstrates that the proposed test is feasible and reliable.