The invention relates to the field of textiles, in particular to yarn of tubular fabric, a preparation method of the yarn, heat-resistant warp-knitted tubular fabric and a protective garment. First yarn is used for weaving the tubular fabric, and second yarn is accommodated in a tubular part of the tubular fabric; the first yarn is prepared from flame-retardant viscose and a first fiber material through blending; the second yarn is prepared from modacrylic fiber and a second fiber material through blending. The first yarn has higher flame retardance. The second yarn has higher flame-retardanceand thermal-insulation properties and thermal expansion performance. The second yarn is embedded in the tubular part, so that the tubular part is firmer and not prone to deformation, the tubular partoutwards bulges, and the thermal insulation effect is enhanced. The protective garment is prepared from the tubular fabric, the bulging tubular part has a buffer function on outside heat, and harm ofheat to a human body is reduced.