In the past decades, the interface between two oxides LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention since a quasi-two-dimensional electron gas (q2DEG) at the interface was observed. It is generally believed that polar discontinuity at polar/non-polar oxide interface is responsible for the emergence of q2DEG at the interface. Recently, how to modulate the q2DEG at the interface is becoming a new research focus. Capping other oxide thin layer on LAO layer is one of alternative approaches to controlling the generation of q2DEG at interface. However the mechanism or origin for tuning q2DEG at capped LAO/STO interface has not yet completely understood. Using the first-principles calculations within the density functional theory, the electronic properties of La1-xSrxMnO3-capped LaAlO3/SrTiO3 heterointerfaces with different doping concentrations of Sr atoms are investigated. The system is composed of four layers of La1-xSrxMnO3 (LSMO), three layers of LAO and four layers of STO, denoted as 4LSMO/3LAO/4STO. The interface is normal to the direction of cubic phase, namely (La1-xSrxO) layer and (MnO2) layer appear alternately at LSMO, and (LaO) layer and (AlO2) layer appear alternately at LAO. In the absence of LSMO layers, q2DEG does not appear at the LAO/STO interface. It is found that the electronic structure of 4LSMO/3LAO/4STO can be tuned significantly by capping LSMO layers. For concentration of doped Sr atoms less than 1/3, a q2DEG at LAO/STO interface is observed. In this case, a strong polarization existing in LSMO, together with the polarization in LAO, forces the electrons to be redistributed, thus inducing the q2DEG at LAO/STO interface. With the increase of the concentration of Sr atoms, the polarization in LSMO becomes weaker and weaker. When the concentration is higher than 1/3, the polaried electric field fails to make the electrons redistributed, thus the q2DEG disappears from interface.#br#Another interesting feature of the present work relates to the distribution of Sr atoms in LSMO. It is found that the electronic structure of 4LSMO/3LAO/4STO changes little with respect to the distribution of Sr atoms in LSMO. The system does not undergo the conductor-to-insulator transition for Sr atoms doping at different sites as long as the concentration of Sr does not change. The reason could be understood as follows. The LSMO layer is in a metallic state, the extra electrons, which are generated due to substituting La with Sr, will be delocalized rather than localized at each doped Sr atom. It is reasonable to expect that the electronic structure of the system should be less sensitive to the specific doping site of Sr in LSMO.