Three series of soft polyvinyl chloride (soft PVC) compounds were prepared at different processing conditions during PVC powder mixing. The processing conditions varied in two different adding procedures of calcium carbonate (CaCO3) powder, two discharge temperatures and two mixing frequencies. The influences of the processing conditions on the glass transition of soft PVC compounds filled with calcium carbonate were extensively investigated by using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and scanning electron microscope (SEM) associated with energy-dispersive X-ray spectroscopy (EDS). It was found that, under the combinative conditions of addition of CaCO3 powder using the one-step procedure instead of two-step procedure, increase of discharge temperature or use of higher mixing frequency, the glass transition of plasticized PVC component could shift to lower temperatures for soft PVC compounds. Under the same formulas, the one-step procedure was confirmed to be preferred, since two separated glass transitions were detected in DSC measurements of the samples prepared by the two-step procedure, where the higher glass transition corresponded to the less plasticized PVC component due to worse mixing efficiency. This finding is quite helpful for the preparation of soft PVC compounds with lower glass transition temperatures.