The construction of urban underground highway tunnel crossing underneath existing metro lines often causes large settlement of existing metro lines and ground surface; and it is significant to study its settlement regularity and control technology. The construction process of a highway tunnel crossing underneath two existing metro lines in Santiago, Chile, which adopts side heading method combined with advance support, primary support without systemic anchor bolt, secondary lining and temporary support, is simulated by software FLAC3D; and the settlement of the existing metro lines and ground surface is analyzed. The numerical simulation results show that the maximum settlements of ground surface and existing metro lines are 35.9 mm and 33.4 mm, respectively; the maximum differential settlement ratios of transversal and longitudinal rail track plane are 0.623‰ and 0.27‰, respectively, and the differential settlement ratio is less than specified settlement threshold; the settlements of the existing metro lines and ground surface are within the limits of specification and requirement of metro operation department; and the structure safety of existing metro lines can be guaranteed during the urban underground highway tunnel construction.