Abstract: This paper presents a new methodfourth law loop switching method,which is based upon the traditional fourth law loop and is suitable for fast acquisition.According to this principle,we develop a FY2 meteorological satellite CDAS raw image demodulator which has the properties of easy designing and adjusting,fast acpuisition,wide capture range and good bit error rate performance.Both testing and actual use prove that this demodulator achieves good specifications and can fully satisfy the actual demands.