Online mental health support plays a crucial role in addressing the mental health issues faced by modern individuals. However, delivering high-quality online mental health support presents a significant challenge. In response to this challenge, we introduce MentalBlend, a framework that leverages psychological counseling theories, including Cognitive-Behavioral Therapy, Dialectical Behavior Therapy, Person-Centered Therapy, and Reality Therapy, to guide large language models (LLMs) in offering professional online mental health support to individuals seeking assistance. Experimental evidence validates that the collaboration between LLMs and the MentalBlend framework results in the generation of responses that align with professional standards for online mental health support. Overall, our research aims to contribute to advancing the capabilities of LLMs in understanding the emotional backgrounds of help-seekers and delivering professional mental health support effectively.