Chemotherapeutics with systemic administration usually suffer from low treatment efficacy and off-target toxicity. In contrary, the photoactivatable cancer theranostics can achieve image-guided precise control of therapeutic dose and location. However, the conventional photoactivatable chemotherapeutics usually require decoration of chemotherapeutics with additional photo-responsive groups through tedious synthetic procedures, which can lead to undesirable toxic byproducts and seriously restrict their applications. Herein, we propose a new strategy for photoactivatable caner theranostics based on photooxidative dehydrogenation reaction, which is only associating with water as the byproduct. To demonstrate the power of this strategy, we utilized the natural dihydrobenzo[c]phenanthridine alkaloids of DHCHE and DHSAN as photoactivatable theranostics to achieve selective imaging and killing of cancer cells by in situ transformation into nucleus-targeted CHE and SAN under light irradiation. Notably, CHE is featured with aggregation-induced emission (AIE) characteristics, which can be used for precise control of the photoactivatable therapeutic dose. This photoactivatable strategy based on dihydroalkaloids is thus promising for precise cancer treatment in clinic.