I'll going to Beijing on next year 24/5 - 6/8 2014, It's a best time to visit or sandstorm happen?

Do you mean from May 24th to June 8th? If so, it is suggested to change your plan. Generally, he sandstorm will happen from mid April to early June, so it may make your trip a little upset. Anyway, you can pay close attention to the weather forecast for due time in the next year. Maybe the situation will be satisfactory for the tour.