The present invention relates to a flexible package-forming machine for horizontal packaging and method of manufacturing flexible packages. The proposed forming machine consists of an unwinding unit (11), a perforating unit (12), a folding unit (14), a welding unit (16), and a cutting unit (17), wherein a compensating unit (13) is further included intercalated between the perforating unit (12) and the folding unit (14), linking a section with intermittent forward movement (S1) of the flexible band (1) where the flexible band moves in an intermittent manner and including the perforating unit, and a section with continuous forward movement (S2) where the flexible band moves in a continuous manner and including the folding unit, flexible band being accumulated in said compensating unit during the forward movement of the section with intermittent forward movement (S1) and the accumulation being reduced during breaks in the forward movement of the section with intermittent forward movement.