The insulation resistance of photovoltaic array unqualified will bring significant accident to the equipment and personal safety, photovoltaic power station need to monitoring insulation resistance of photovoltaic array automatically. Therefore, this paper proposes a new insulation monitoring scheme based on unbalanced electric bridge, and has carried on the theoretical derivation and the error analysis. Practical operation shows the scheme can accurately detect the size of the insulation resistance value of the DC bus and branch of photovoltaic array, and improve the security and stability of the photovoltaic power station operation.