Input signal power of GPS receivers changes with time. If fixed threshold is used to determine completion of the acquisition stage, two problems will arise, namely, decreasing in detection probability or increasing in false alarm probability. Therefore, it is proposed to apply a decision method for adaptive acquisition threshold in signal acquisition. Specifically, maximum likelihood estimation method is used on the sample data for decision statistic to obtain the maximum likelihood estimation value for noise variance. Afterwards, adaptive acquisition threshold can be obtained based on the Constant False Alarm Rate (CFAR). As the average acquisition time is one of the most important indexes of GPS receivers, calculation methods for the average acquisition time are analyzed for the case when both carrier frequency and PN code phase are uncertain. Furthermore, constraint relationship among acquisition time, Signal-to-noise Ratio (SNR) and threshold are derived. Analysis results show that adaptive acquisition threshold method can reduce acquisition time effectively, thus accomplish faster acquisition.