In the field of complex optical systems, erbium doped fiber ring laser(EDFRL) is a kind of imperative fiber laser. Hundreds and thousands of longitudinal modes can typically coexist in this laser system, exhibiting nonlinear mode dynamics such as complex mode hopping and high-dimensional chaos. In this paper, dynamical output in free-running multi-longitudinal mode(MLM) EDFRL has been investigated experimentally and simulated numerically. Method to obtain real-time frequency and temporal intensity dynamics in group of dense individual longitudinal modes around central wavelength 1546.767nm is based on heterodyne detection, the utilized beat signal is generated by optical interference of adopted laser and reference laser. Frequency dynamics and time evolutions of heterodyne signal can be recorded by a radio frequency analyzer and fall graph. According to the experimental results, the output of MLM EDFRL mode dynamics is detected and the analysis for mode intensity can be obtained. Trying to simulate and explain the experimental observations, MLM laser model based on the saturable absorption of ions and ion-pairs in the gain medium, together with cross-coupling operation of corresponding longitudinal modes, has been discussed. The MLM laser model can effectively describe the individual behavior and clustering behavior of a large number of longitudinal modes, and reproduce theoretically the multi-mode dynamics of EDFRL system in frequency domain.