The invention discloses a halogen-free flame retardant and a preparation method thereof and a flame-retardant polyolefin composition composed of the same. The structural formula of the halogen-free flame retardant is shown in the description, wherein the ratio of x to y to z is 1:(5-10):(1-5). The preparation method comprises the steps that melamine, phosphoric acid and piperazine are weighed and put into a high-speed mixer of which the top is provided with a liquid spray device to be stirred and mixed to be uniform, and a halogen-free flame retardant premixed material is obtained; the halogen-free flame retardant premixed material is subjected to a high-temperature dehydration condensation reaction, and the polyphosphoric acid-melamine-piperazine composite salt halogen-free flame retardant is obtained. The halogen-free flame retardant with the composite salt structure has the advantages that the halogen-free flame retardant is good in heat stability, high in phosphorus content and free of impurities such as halogens and inorganic salts, and the index of whiteness is larger than 90; few synthesizing process steps are needed, operation is simple and easy to carry out, the requirement on equipment is low, and the product yield reaches up to 80% or above; the flame-retardant polyolefin composition containing the halogen-free flame retardant has the good flame-retardant property, high water-resistant and heat-resistant properties and the excellent pigmenting property.