A separation membrane support substrate composite of a laminated nonwovens fabric with a front coating layer, a middle layer and aback layer which are formed by heat bonding and subjected to calendered treatment has been patented. The front layer contains thermoplastic resins witha single filament diameter of 7-30um. The middle layer contains a meltblown fibers and a fiber basis weight of at least 1 gsm and no more than 30% of the materialÍs total basis weight.The back layer has at least one layer comprising thermoplastic resin filaments with a single filament diameter of 7-20 um and a fiber basis weight of 3-40 gsm. The laminated nonwoven fabric has an apparent density of 0.67-0.91 gsm; the laminated nonwoven fabric has a thickness of 45-110 um and the smoothness of the coating layer surface is 0.2-2 um in terms of the KES surface roughness SMD. The thermoplastic resin must be the same in the front and back layer; a flat roll is used for heat bonding at a temperature of 50-120ÁC below the melting point of the thermoplastic resin filaments after which the calendar treatment is carried outat a temperature of at least 10ÁC, higher than the temperature of the heat bonding and lower than the melting point of the thermoplastic resin filaments.