The biuret polyisocyanate is obtained through the reaction between at least one kind of diisocyanate, selected from aliphatic diisocyanate and alicyclic diisocyanate, and biuret. The biuret polyisocyanate contains carbamide linkage compounds as intermediate product of the reaction between diisocyanate and biuret including urea dimer. The urea dimer content is less than 0.5 wt% of the total product weight. Said biuret polyisocyanate which is measured by using sample cell with length of 2 cm at 430 nm has at least 90 % light transmittivity. The said biuret polyisocyanat preparing process is also disclosed.