Context. The solar wind transients propagating out in the inner heliosphere can be observed in white-light images from Heliospheric Imager-1 (HI1), an instrument of the Sun Earth Connection Coronal and Heliospheric Investigation on board the Solar Terrestrial Relations Observatory (STEREO), from two perspectives. The spatial velocity distribution inside solar wind transients is key to understanding their dynamic evolution processes. Aims. We generated a velocity map of transients in 3D space based on 2D white-light images and used it to estimate the expansion rate as well as some kinematic properties of solar wind transients. Methods. Based on the recently developed correlation-aided reconstruction method in our previous work, which can recognize and locate 3D solar wind transients from STEREO/HI1 image data, we further developped a new technique for deriving the spatial distribution of the radial velocities of the most pronounced features inside solar wind transients. Results. The technique was applied to events including a coronal mass ejection (CME) and three small-scale transients, so-called blobs, observed by HI1 on 3–4 April 2010 to reconstruct their radial velocity maps. The results match the forward-modeling results, simulations, and in situ observations at 1 AU fairly well. According to the obtained spatial distributions of height and radial velocity of the CME, we analyzed the self-similarity of the radial expansion of the CME ejecta. The dimensionless radial expansion rate of the northern and middle parts of the CME ejecta varies in the range of 0.7−1.0 at heliocentric distance between 25 R_⊙ and 55 R_⊙ and the rate of the southern part in the range of 0.3−0.5, suggesting that the CME structure was distorted and shaped by the ambient solar wind. The technique we developed is expected to be applied to more events.