 This is a phase II, placebo-controlled, randomized, double-blinded clinical trial. Study objective is to assess the efficacy and safety of mirtazapine in advanced NSCLC patients with malignant tumor related depression. Study hypothesis is that advanced NSCLC diagnosed with depression undertaking palliative chemotherapy with mirtazapine treatment for 8 weeks will have remarkable improvement in depression compared to baseline. Eligible advanced NSCLC Patients with PHQ-9 score 8, and undertaking palliative chemotherapy will be enrolled into this study. patients will be stratified gender, age, Numerical Rating Scale score for cancer pain 0-34-67-10 randomized 1:1 into mirtazapine or placebo treatment. Patients in mirtazapine arm will be orally administered with mirtazapine 15mg, QD, consecutive medication for 8 weeks; along with palliative chemotherapy regimen decided by investigators. Patients in placebo arm will be orally administered with placebo 15mg, QD, consecutive medication for 8 weeks; along with palliative chemotherapy regimen decided by investigators. During the treatment, Patient health questionnaire PHQ-9, Hamilton Depression Scale HAMD-17 and European Organization for Research on Treatment of Cancer EORTC quality of life questionnaire-C30 QLQ-C30 questionnaires will be collected at baseline, 3 weeks d22 and 8 weeks d57, or treatment discontinuation date due to depression deteriorated or suicidal tendency and behavior. Follow-up will last up to 4 weeks after treatment end with depression assessment questionnaires every 2 weeks. Study endpoints: primary endpoint is the anti-depression efficacy response rate. Response defined as the PHQ-9 or HAMD-17 questionnaire score decrease 50 compared with baseline level. 