In the aim of preparing cotton straw-expanded polystyrene (EPS) composite block with good mechanical properties, we probed into the influence of straw form on the physico-mechanical properties of the block. The form of straw was classified as particle, chip, and fiber according to length and particle size of straw. Result showed no significant effect on the early mechanical properties of block by straw form. However, when the mix proportion was fixed and the length and size was small (particles), the block of 28-day age or above had the largest compression strength, anti-splitting strength and axial compressive strength, reaching 6.95 MPa, 0.93 MPa and 6.88 MPa, respectively; when the particle size of straw increased (chips), the compression strength, anti-splitting strength and axial compressive strength of the block declined; when the length of straw increased (fiber), the flexural strength and elastic modulus of the block were the largest, amounting to 1.44 MPa and 80.42 MPa, respectively; but the elongation of straw fiber also instigated the water absorption of blocks. In conclusion, the form of straw has remarkable influence on the physico-mechanical properties of cotton straw-EPS block. We should select proper straw form according to specific engineering requirement.