In order to determine two endpoints (P₁ and P₂) in the straight-line segment of stress-strain curve of plastic concrete, and to obtain the elastic modulus, we carried out uniaxial compression test on 48 concrete specimens of four different mix proportions. Through analyzing typical stress-strain curve, we selected approximated straight-line in the curve, and compared the secant modulus with mean value of tangent modulus. On this basis, we studied how to determine P₁ and P₂. The results show that 1) the approximated straight-line segment appears at the ascent stage in the stress-strain curve of plastic concrete; 2) with given strength of plastic concrete less than 5 MPa, it is suitable to choose 60% and 80% of peak stress as the elastic modulus of P₁ and P₂, respectively, otherwise, further study is required. Finally, the research offers a method for determining the values of material performance of plastic concrete with low strength.