Through diagnosing plasma density and calculating the intensity of microwave electric field, four 10cm electron cyclotron resonance (ECR) ion sources with different magnetic field structures are studied to reveal the inside interaction between plasma, magnetic field and microwave electric field. From the diagnosing result it can be found that the plasma density distribution is controlled by the plasma generation and electron loss volumes associated with magnetic field and microwave power level. Based on the cold plasma hypothesis and diagnosing result, the microwave electric field intensity distribution in the plasma is calculated. The result shows that the plasma will significantly change the distribution of microwave electric field intensity to form a bow shape. From the boundary region of the shape to the center, the electric field intensity varies from higher to lower and the diagnosed density inversely changes. If the bow and its inside lower electric field intensity region is close to the screen grid, the performance of ion beam extracting will be better. The study can provide useful information for the creating of 10cm ECR ion source and understanding its mechanism.