The invention belongs to the field of polymer foams and discloses a self-flame-retardant modified melamine-formaldehyde foam and a preparation method thereof. According to the method, by flexible chain segments of polyvinyl alcohol (PVA) and a special process, polyvinyl alcohol is reacted with melamine formaldehyde so that the flexible chain segments of PVA are embedded between rigid radicals of melamine formaldehyde and the characteristics that the foam is easy to embrittle and chalk and the toughness is poor under the condition meeting the need of flame retardancy are achieved. The melamine-formaldehyde foam prepared by the preparation method has the advantages of good flexibility, certain compressive strength, oxygen index greater than 36% and elastic recovery rate up to 90%.