A novel design of the Depleted P-Channel Field Effect Transistor (DEPFET) with non-linear response is at the heart of the 1 Mpixel DSSC camera (DEPFET Sensor with Signal Compression) currently being developed for ultra-fast imaging of soft X-rays at the European XFEL. The calibration of the 1 Mpixel DEPFET is the key to reach both single-photon resolution and high dynamic range but also the major challenge. The calibration process is based on two steps: i) the calibration of the linear response at low intensity by means of hard X-ray lines and ii) the calibration of non-linear response up to the maximum energy range. For the latter, we conducted a dedicated beam time with soft X-rays at the SQS beam line of the European XFEL.The aim of this work is to discuss the general calibration strategy and to present the experimental results of the calibration campaign on the first DEPFET ladder. It will focus on the measurement technique, modelling and parametrization of the non-linear response. The achieved results show achievement of noise levels below 20 electrons rms and an input range of several MeV per pixel per pulse.