Facing the challenges of global warming, energy scarcity, and energy price fluctuations, many countries consider energy security to be a major part of energy policy and have started to develop relevant strategies. This study applies multi-dimensional energy security indicators to review the performance of Taiwan’s energy security related measures and to analyze the impacts of the current energy policy on Taiwan’s energy security. The results show that Taiwan’s current energy policy is effective in improving the security of the energy supply. Taiwan’s dependence on imported energy is rather high, however, which makes Taiwan vulnerable to international energy price fluctuations and liable to suffer from increased energy costs. The uncertainty of energy efficiency improvement will hence affect the expected results of the energy security. Facing the risk and uncertainty inherent in social and economic development, many countries have adopted an energy supply diversification strategy in their energy security policy to control the energy import risk and cost to ensure that sufficient energy can be obtained at an affordable price level. Diversification helps the energy system respond to external changes and shocks. Diversification can also lessen the vulnerability of a single energy source to supply shocks and the market power of various energy supply sources. This study develops the “diversity reliability index” and “co-vary diversity reliability index” based on the Hirschman-Herfindahl and Shannon-Wiener indices, investigates the contribution of different energy sources to the energy system, and analyzes the covariance between various energies and their impacts on energy security to determine the impact of energy diversity in reducing the risk of energy supply shortages and cost fluctuations. This study also used “co-vary diversity reliability index”, which includes multi-objective model of 3E to simulated the electricity and energy structure from 2015 to 2030 by different scenarios of expand renewable energy development, achieving carbon mitigation goal and implement of nuclear free homeland policy. And effects on growth rate of GDP of the general economy, greenhouse gas emission, electricity costs, energy consumption, and the variance of energy security indicators.