The invention discloses a synthesis method of a rare earth / iodine co-doped photo-degradation catalyst. The synthesis method comprises the following steps: stirring and quickly dissolving iodic acid, adding deionized water, dropwise adding tetra-n-butyl titanate (Ti(OBu)4) slowly, and producing foam precipitate; after the tetra-n-butyl titanate is completely dissolved in water, performing ultrasonic dispersing on the tetra-n-butyl titanate until the tetra-n-butyl titanate is dispersed uniformly, reacting at 70 to 90 DEG C to obtain milk white gel, slowly decreasing the temperature after the gelis dried, and preparing to obtain a powder I / TiO2 precursor; roasting the powder at 380 to 420 DEG C, and cooling; dropwise adding rare earth nitrate solution slowly with continuous stirring, stirring uniformly, performing ultrasonic dispersing, and then evaporating water in a water bath at 70 to 90 DEG C; drying in a drying box, and roasting at 340 to 360 DEG C in a muffle furnace to obtain a light yellow powdery rare earth / I / TiO2 photocatalyst. The synthesis method of the rare earth / iodine co-doped photo-degradation catalyst is simple in preparation process and lower in cost; rare earth isdoped in titanium dioxide, which is beneficial to improving the photodegradation activity of a catalyst.