We present the morphology of the polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer and polystyrene homopolymer (PS homo) blends at the air/water interface. Based on the morphology, we construct a phase diagram using two variables, sum of PS mass in PS-b-PEO and mass of PS homo over total polymer mass, φ, and the ratio of molecular weight (Mn) of PS homo to Mn of PS in PS-b-PEO, α. In general, we find out both the increase of domain size and morphology transition from circular to cylindrical to planar morphology as φ increases, and only the similar morphology transition is seen as α goes up. Although this is observed in a typical morphology of PS-b-PEO as the relative PS block ratio and spreading concentration increase, our blends have two different features: 1) we can adjust the morphology easily by controlling the amount and Mn of PS homo; 2) New morphology that we call melt morphology is discovered for small α as PS homo act like plasticizer.