The invention relates to a profile modifying / water plugging agent for an oil well, in particular to grafted polymer microspheres for oil displacement modification and a preparation method thereof. The grafted polymer microspheres are prepared from etherified starch, acrylamide, N,N-methylene bisacrylamide and ammonium persulfate in the mass ratio of (1-2) to (0.5-1.5) to (0.04-0.2) to (0.1-0.5). The polymer microspheres have the advantages of controllable particle size, high homogeneity, high temperature resistance, high salt resistance, wide raw material source and low manufacturing cost, is not hydrolyzed in a severe saline environment, and is non-degradable; starch microspheres can further go deep into an oil reservoir after being swelled by absorbing water; meanwhile, the polymer microspheres have the rigid characteristic of starch micro molecules and the flexible changeability characteristic of the acrylamide, have high deepening and blocking performance, and are environmentally friendly; by adopting the grafted polymer microspheres, the non-homogeneity of a rock stratum is lowered, and the crude oil recovery factor is increased.