The invention discloses a novel mould-proof silane modified polyether sealant and a preparing method thereof. The sealant is prepared from, by weight, 80-110 parts of silane modified polyether resin,180-200 parts of activated nanometer calcium carbonate, 40-60 parts of plasticizer, 2-5 parts of titanium dioxide, 0.4-2 parts of heat stabilizer, 2-4 parts of dewatering agent, 0.3-0.5 part of mould-proof agent, 1-3 parts of coupling agent and 0.5-2 parts of catalyst. The plasticizer is one of or the composition of more of di-iso-decylphthalate, diisononyl phthalate, di-isononyl-cyclohexane-1,2-dicarboxylate, polyether polyol N220 and polyether polyol N330. The mould-proof agent is dithio-2,2-bis(N-methylbenzamide). By means of the prepared silane modified polyether sealant, the mould-proof performance is improved, the service life is prolonged, surface drying and viscosity eliminating are rapid, and high elongation and high yellowing resistance are kept. The method is simple in preparation, feasible in process, low in production cost, low in energy consumption and beneficial for industrial production and has good economic benefits and social benefits.