Differential phase-contrast computed tomography (DPC-CT) is a novel x-ray inspection method. Currently, DPC-CT reconstruction problems are solved by using parallel-beam, fan-beam and cone-beam algorithms. The above algorithms cannot show the internal structures of rod-shaped objects well enough for only reconstructing a few slices. The helical cone-beam algorithms have significant advantages for rod-shaped objects over other algorithms. Along with our numerical evaluation and verification, we report a PI-line-based approximate algorithm for helical cone-beam DPC-CT, which can be applied to reconstruct the refractive index decrement distribution of the samples directly from phase-contrast projection images. Simulations using a 3D Shepp-Logan phantom are carried out to verify the proposed algorithm. Reconstruction results show that the proposed algorithm can provide higher quality performance compared with the existing interpolation-based reconstruction algorithm.