美国申请博士难吗？
PhD application is very competitive， for top universities in USA. The competition is due to the limited number of positions and the large amount of applicants (there were around 30k applications last year and there are 20k new international students enrolled into US Graduate schools this year). For some fields/programs， the competition can be as high as 1-in-5 or even lower. So if you want a PhD， make sure that your qualifications are at least comparable with other applicants who have high chance of acceptance because any additional qualification will help！
For most cases， students who do not need to obtain a visa to study in the US but only need a F-1 student visa can start their Master’s studies from September/October（depending on schools）. This allows them to pay less money on accommodation， and it is also convenient to arrange their family and work schedule. Most of my friends would like to complete their MS before starting their PHD so they are able to apply “full stack”（master's degree+phd degree）for an OPT extension which would allow them to stay in the United States up to 36 months after graduation（If you are applying to a Masters program， you may be eligible for one year of optional practical training（OPT）after completing your program. This allows you to remain in the U.S. and continue your career development）.
The downside of doing master's first is the time limit. You have 8 years total after you arrived in the U.S. to get your Bachelor's /Master's Degree plus any Additional Credits towards Doctorate or Professional degrees. Otherwise， you will have to leave the country and reapply once your F-1 Visa expires 。
Some students may take long breaks between undergraduate and graduate studies， especially those who wish to pursue their graduate degrees outside the U.S. It depends on each school's policy about accepting transfer credit， but in general， 40 to 60 credits earned toward the bachelor's degree is recognized by many American colleges and universities toward a graduate degree. In exceptional ways， courses taken while studying abroad can be transferred back to the U.