The invention discloses a sea-island fibre and a preparation method thereof. A PTT (polytrimethylene terephthalate) slice is taken as an island component, a COPET (copolyester) slice is taken as a sea component, the two raw materials are respectively dried, extruded and molten, then the melts are fed into a spinning box and then are extruded out after being accurately metered by a metering pump, the island component is uniformly distributed into the sea component, the island component and the sea component are sprayed out from a same spinneret orifice, then cooling, moulding, oiling, drawing based on a silk guiding roller, winding and silk doubling are carried out to obtain a PTT sea-island fibre, the breaking strength of the fibre is 2.8-3.7cN / dtex, the breaking elongation is 57-62%, and the evenness uster value is less than 1.3%; and the filament number of the island component PTT is 0.05-0.20DPF. The sea-island fibre product not only has the characteristics of a common sea-island fibre but also has the characteristics of high elasticity, lodging resistance, high dyeing degree, high colour fastness, bright finished product colour, no wrinkling, easy care and memory function, thus the sea-island fibre is an ideal fibre used for a top-grade garment material and has great market potential.