Abstract: In order to identify and justify the conditions for discriminating safe spanning length of an underwater pipeline, a model experiment is designed according to the elastic force-gravity similarity theory. The pipeline is fixed at both ends, having an angle with water flow of 90°, 75°, 60°, 45° and 30° respectively and freely spanning for 1.00 m, 0.81 m, 0.73 m and 0.70 m respectively. During the experiment, the free spanning pipeline is discriminated for vibration under 3 conditions, i.e., f_n ≥10 f_s, f_n ≥ (0.7)^-1f_s and f_n = 5 f_s (f_n is the natural frequency of free spanning pipeline, f_s is the vibration frequency of water flow). The results show a safe and reasonable critical length at f_n = 5 f_s (i.e., the reduced velocity V_r = 1). The critical lengths are plotted under the condition of f_n = 5 f_s, depending on diameter, wall thickness and flow rate. This study can provide references for safety assessment of free spanning pipeline underwater.