The deep space optical communication links are very far away, so a very harsh power budget is required. Since there are many factors affecting the feature of communication link, a very complex trade-offs consideration is often given to the power budget. The paper emphatically analyzes the link distance, optical antenna diameter, beam divergence, atmospheric channel loss, optical system loss, detector sensitivity and other factors affecting the link equation, signal to noise ratio (SNR) of detector and bit error rate (BER) of communications. Simultaneously the paper proposes a new method of budgeting transmission power combining the link equation, SNR and BER. An outside demonstration experiment for the ground-ground optical communication of 7km, is carried out, the losses of the atmospheric channel and optical system are tested. Then again many experiments in indoor labs, test the beam divergence and detector sensitivity are performed. The experimental results and theory analysis show that the budget method is feasible in application to the deep space optical communications.