A varifocal f-theta lens was designed for Nd：YAG laser making scan focusing system,so that it apply to differ making area but change a lens necessarily.The system consists of four lens,made by glass BK7,SF18 and SF11.The total length of the system is 122.96mm,working at 1064m.Using the minus aberration of f-theta lens and machine zoom principle,the three alterable focus system was designed.The two effective focal lengths of the system are 350mm and 400mm,which are usually used in f-theta lens.The scanning field is 220mm×220mm,250mm×250mm,respectively.All of the scanning angles are±18°.For these two effective focal lengths,the spots on the image surface at zero view field,0.7-view field and full view field are all inside the airy disc.The system MTF curve of each effective focal length are accordance with the corresponding diffraction limits curve.The biggest wave aberration is smaller than 1/4 wave lengths.The maximum field curvature and maximum f-theta system distortion respectively satisfies the precision demand of the laser marking machine in China.