Assessment in senior high EFL writing is often dominated by exam-facing criteria that privilege linguistic accuracy and fixed forms, which can misalign with project-based learning (PBL) writing, where students compose texts for authentic purposes and audiences. This paper presents an audience-oriented assessment system (AOPWAS) designed to align assessment with PBL writing processes and outcomes while remaining feasible for secondary classrooms. Grounded in assessment-for-learning and feedback literacy scholarship, the system integrates (a) an audience-aware analytic rubric, (b) portfolio-based evidence mapping across drafting and publication, (c) structured peer and self-co-assessment routines, and (d) brief teacher-student conferencing focused on feedforward and revision decisions. Rather than relying on complex statistical modelling, the protocol adopts a qualitatively driven mixed-evidence approach: qualitative data (classroom observation notes, student reflections, interview prompts, and writing portfolios) are prioritized to understand how assessment criteria shape revision behaviour and motivation, while lightweight descriptive summaries (e.g., rubric dimension profiles, rater agreement checks) support transparency. The paper details design principles, assessment tools, step-by-step classroom procedures, and an evaluation plan that can be adapted to different PBL writing topics. It contributes a practical assessment blueprint for teachers seeking to reduce assessment-instruction mismatch and to cultivate students’ audience awareness, feedback uptake, and self-regulated revision in PBL-based writing.