Filamentous fungi are widely used in the field of recombinant protein expression due to their well-established protein modification systems and excellent secretion capacities. Although Penicillium oxalicum has been developed as an expression host, its potential for efficient and convenient protein production has not been fully exploited. In this study, we obtained an engineered strain by dominant activation of the G protein PGA3 using a point-mutation method based on the low extracellular background P. oxalicum host Δ13A-OamyR. This genetically modified strain, OamyR-QL, with faster cell growth and a more efficient Pamy15A promoter, will be used to construct a novel expression system. The relevant genes and pathways involved in the response to the G protein dominant activation in the engineered strain were revealed by RNA sequencing. Moreover, the transcription activator AmyR was overexpressed in OamyR-QL, resulting in a dramatically enhanced efficiency of the Pamy15A promoter. The construction of an efficient, low-background system by utilizing the G protein-AmyR regulatory pathway provides not only a theoretical reference for the genetic engineering of other filamentous fungal strains, but also a preferable option for the efficient and high purity expression of recombinant proteins in filamentous fungi.