The basic energy detector still plays an important role in the extended target detection, with the advantage that it does not depend on any prior knowledge of the observed target. However, for the complex extended target model, the detector's performance has not been adequately studied. This paper will focus on the theoretical detection performance of energy detector for the fluctuating extended target in the white Gaussian noise. Herein the target's distributed condition is described by a density factor and three Swerling fluctuation cases are also taken into account. With the aid of moment-generating function approach, we present the detailed forms of the test statistic's probability density function under different model assumptions and then obtain the corresponding detection probability. Whereafter, the influences of the target model on the detector's performance are further discussed and verified with some simulation results. This study can be used to sustain the design of an effective adaptive energy detector.