Accurate modeling of streamer branching and calculation of space-charge distributions are of critical importance for advancing lightning attachment mechanisms. An electrostatic model for branched streamers in long air gaps is constructed based on the Charge Stimulation Method in this paper. Under quasi-static conditions with constant axial channel field and surface-confined space charge, streamer branches were explicitly described as conductive channels employing internal point charges for hemispherical heads and linearly distributed line charges for cylindrical channels. Electric fields were computed through dynamic charge configuration, enabling derivation of branch velocities via Meek’s theory. In simulations of 290 kV lightning impulses in 0.57–1.0 m rod–plate gaps, the model successfully reproduces quasi-ellipsoidal branching envelopes, complete propagation velocity profiles, and the step-rise features of the local electric field. Experimental validation demonstrates that the present model overcomes the limitations of traditional macroscopic space-charge formulations, which neglect branching morphology and cannot predict propagation speed or internal field strength. It provides a high-precision numerical tool for lightning-attachment assessment and leader development studies.