Abstract: The design of the first undu1ator UD1 in NSRL is introduced. The design parameters and main technical requirement are given. The quality requirement on undulator magnet field are analysed considering demands of light users as well as that of the electron beam stability. It is given that the first and second integral of the magnet field should be less than 2×１０－４Ｔ·m and 2×１０－４Ｔ·m2 respectively, the phase error should be less than 10°.