The invention relates to novel room-temperature vulcanized silicon rubber and a cross-linking agent thereof, and discloses application of a phosphonitrilic chloride trimer modifier and a hydrolytic condensation product of the phosphonitrilic chloride trimer modifier as a cross-linking agent in the room-temperature vulcanized silicon rubber. The phosphonitrilic chloride trimer modifier and the hydrolytic condensation product of the phosphonitrilic chloride trimer modifier comprise siloxane structures and phosphonitrile groups, wherein the weight ratio of siloxane groups to phosphonitrile groups is 100:(10-60) along with different hydrolytic condensation degrees of the phosphonitrilic chloride trimer modifier and the hydrolytic condensation product of the phosphonitrilic chloride trimer modifier. The novel room-temperature vulcanized silicon rubber provided by the invention is stable in property, the use environment of the novel room-temperature vulcanized silicon rubber is widened, particularly compared with a similar product, prepared flame-retardant silicon rubber is good in mechanical property, and the mechanical strength of a product can be well improved; the prepared silicon rubber is good in flame retardancy, and the limit oxygen index of the silicon rubber is increased.