A new method of noise analysis in RF (radio frequency) receivers is provided when the system modeling is required while only some limited parameters of the whole RF system are known. The analysis procedure of AWGN(additive white Gaussian noise) through noisy linear circuits is given in detail using the theory of equivalent computation of noise power density. Take a communication station as an example for noise computation and system modeling. Noise parameters of each component such as noise figure in the RF receiver are derived in the condition of limited system parameters of the receiver including SNR (signal noise ratio), sensitivity, RF gain etc. Modeling and simulation are carried out to validate the analysis method and fair agreement between the theoretical and simulated results is obtained. This equivalent computation method of noise power density has a good instructional meaning to noise analysis and proper system modeling of RF receivers.